Port to Gtk 4

[danirabbit]

Fixes #1590
Fixes #1795
Fixes #1940

I've tried to comment all non-obvious changes here. A lot of the diff is just minor API changes. Hide whitespace changes might be useful since there's a few try/catches removed or null or type checks added

Categories:

• css: Fixes for some changes to how Gtk.CSS renders gradients

Application:

• Use new method for window save/restore size
• New way to check focus
• No more destroy, connect to close

Package:

• It looks like we never copied remote icons before? But now this causes a crash when Gtk tries to set a Paintable from a File. So removed for now since this does nothing already

StripeDialog:

• Spinbutton no longer has activates_default so use a key controller
• Spinbutton no longer has primary_icon_name so parent a Gtk.Image

MainWindow:

• Leaflet handles mouse back button, so no need to do it ourselves

AppInfoView:

• There was some overcomplicated arrow mouse event stuff here. afaict the motion controller hides and shows arrows more accurately in this branch than master with fewer signals
• I used Gtk.Picture for screenshots instead of Gtk.Image which seems to obsolete Use DrawingArea for screenshots #1870

AbstractAppContainer:

• Made overflow visible in a revealer here so that button shadows don't get clipped

[danirabbit]

@leolost2605 since AppCenter is somewhere you touched recently could I get a review here? I'd like to do some more UI work but I really want to land GTK4 porting first :)

[leolost2605]

A few things I noticed during testing, I think most of them aren't blocking though

• If you've got only a few packages the packages grid in the update view is bottom aligned but I think that's also present in master

• Also I'm being greeted by a restart required infobar
  

• The backbutton doesn't have the back style applied but I've seen the same thing in settings so I think that's a granite/stylesheet issue?

• Switching between remotes plays an leaflet animation

• There are a lot of deprecation warnings but I guess those can be fixed in follow up PRs

[danirabbit]

• If you've got only a few packages the packages grid in the update view is bottom aligned but I think that's also present in master

Yup that's in master too. Definitely wanna iterate here in a future branch

• Also I'm being greeted by a restart required infobar

Ah because widgets are visible by default in GTK4 and we forgot to remove this when we removed the updates recently in another branch. Fixed!

• The backbutton doesn't have the back style applied but I've seen the same thing in settings so I think that's a granite/stylesheet issue?

Yeah, pushed a fix to stylesheet 😅

• Switching between remotes plays an leaflet animation

Yeah this might be a leaflet problem 🤔 When we move to NavigationView there's a property there for disabling animations, but I was trying to keep from blowing up the diff. Is this okay to fix in a follow up?

• There are a lot of deprecation warnings but I guess those can be fixed in follow up PRs

Yeah I was trying to keep the diff here as small as I can

[leolost2605]

Seems good enough to iterate on! 🎉

[danirabbit]

Let's fucking goooo