[UI/UX] `.png` -> `.svg` icon overhaul checklist

[yioannides]

Relevant topics: #2091 #2120 #2121

This is my own to-do checklist for all the remaining .png icons, to be remade in scalable .svg format:

• adjustments-autolevel.png
• adjustments-blackandwhite.png
• adjustments-curves.png
• adjustments-huesaturation.png
• adjustments-invertcolors.png
• adjustments-levels.png
• adjustments-posterize.png
• adjustments-sepia.png
• edit-selection-erase.png
• edit-selection-fill.png
• edit-selection-invert.png
• edit-selection-offset.png
• effects-artistic-inksketch.png
• effects-artistic-oilpainting.png
• effects-artistic-pencilsketch.png
• effects-blurs-fragment.png
• effects-blurs-gaussianblur.png
• effects-blurs-motionblur.png
• effects-blurs-radialblur.png
• effects-blurs-unfocus.png
• effects-blurs-zoomblur.png
• effects-color-dithering.png
• effects-distort-bulge.png
• effects-distort-dents.png
• effects-distort-frostedglass.png
• effects-distort-pixelate.png
• effects-distort-polarinversion.png
• effects-distort-tile.png
• effects-distort-twist.png
• effects-noise-addnoise.png
• effects-noise-median.png
• effects-noise-reducenoise.png
• effects-photo-glow.png
• effects-photo-redeyeremove.png
• effects-photo-sharpen.png
• effects-photo-softenportrait.png
• effects-photo-vignette.png
• effects-render-cells.png
• effects-render-clouds.png
• effects-render-juliafractal.png
• effects-render-mandelbrotfractal.png
• effects-render-voronoidiagram.png
• effects-stylize-edgedetect.png
• effects-stylize-emboss.png
• effects-stylize-outline.png
• effects-stylize-relief.png
• image-flatten.png
• image-resize-canvas.png
• image-resize.png
• layer-import.png

@cameronwhite and @pedropaulosuzuki , please let me know what icons are no longer needed so I can remove them from the list!

[cameronwhite]

• I think we could remove addins-manage, help-bug, help-translate, view-rulers, view-zoom-100, view-zoom-select, view-zoom-window. They're either completely unused, or are only referenced by menu items (like the Help menu) that don't create history items or show the icon anywhere

• I'm not sure if view-grid is needed. This is only being used as the IconName for the canvas grid dialog's GtkWindow, but I don't know if that actually does anything. On macOS at least, there's no icon shown for modal dialogs like that

A couple other things noted while going through these:

• along with those four edit-selection-* icons, there is also ui-deselect-symbolic being used for the Deselect action if you're looking to make those all look consistent
• help-website-symbolic is sort of unused (we don't need it for the Help / Website menu), but it's also being reused as the icon for the Fill tool's Global flood mode

[yioannides]

I finished the adjustments- icons!

Few things to note:

• I avoided using white for adjustments-blackandwhite, cause it'll be difficult to handle on light theme and then black on dark mode respectively, so it's best to allude to it.
• We will have to adjust the colors on adjustments-curve to have a light and dark mode, as per all colored elements on GTK apps (eg. --blue-3 on light, --blue-1 on dark), so we'll need to discuss that with @pedropaulosuzuki when the time comes
• The adjustments-invertcolors comes with the Adwaita icon library, so we can use that
• There's no standard icon for the posterize effect (apparently Adobe uses a funnel icon for it, which I don't quite get to be honest), so I made my own which best describes the intention of this effect
• The actual color for sepia (hex #704214) is very difficult to use on a symbolic icon for both light/dark mode, so again, it's best to use a color that alludes to it (--brown-2 on light mode, --brown-1 on dark)

If it's all good so far, I will move on to the other ones next!

[yioannides]

Okay, the edit-selection-* icons, also done! Also redid the ui-deselect icon to blend with the others.

Would you prefer if the "x" in erase is red or do we leave it as is? Let me know and I can change it at any time!

ORIGINAL	w/ RED

[pedropaulosuzuki]

Okay, the edit-selection-* icons, also done! Also redid the ui-deselect icon to blend with the others.

Would you prefer if the "x" in erase is red or do we leave it as is? Let me know and I can change it at any time!

Those look really good! I'd say we go with the Red X version, though the X maybe could be slightly bigger, I think.

[pedropaulosuzuki]

I finished the adjustments- icons!

Few things to note:

* I avoided using white for `adjustments-blackandwhite`, cause it'll be difficult to handle on light theme and then black on dark mode respectively, so it's best to allude to it.

* We will have to adjust the colors on `adjustments-curve` to have a light and dark mode, as per all colored elements on GTK apps (eg. `--blue-3` on light, `--blue-1` on dark), so we'll need to discuss that with [@pedropaulosuzuki](https://github.com/pedropaulosuzuki) when the time comes

* The `adjustments-invertcolors` comes with the Adwaita icon library, so we can use that

* There's no standard icon for the `posterize` effect (apparently Adobe uses a funnel icon for it, which I don't quite get to be honest), so I made my own which best describes the intention of this effect

* The actual color for sepia (hex #704214) is very difficult to use on a symbolic icon for both light/dark mode, so again, it's best to use a color that alludes to it (`--brown-2` on light mode, `--brown-1` on dark)

If it's all good so far, I will move on to the other ones next!

I think blackandwhite and invertcolors look very similar. Also invertcolors has a droplet icon, just like the gaussianblur (which I don't quite like the droplet usage either). Maybe invertcolors could look something like this? (I'm sure you'll get a better idea than what I presented, but I'm just kicking things off)

The others are really good! Specially the levels/curves!