Use the portal

[leolost2605]

• Pretty much same functionality
• Automatically closes after taking screenshots which is the default use case I think
  - Looses the command line options since we can't tell the portal what to do. We could keep the screen option but I don't really see a big use case for that.
• Launching via app actions will now still show the save dialog

Fixes #283

[danirabbit]

Hm I'm not sure about losing the actions. That was the reason I decided not to keep going with moving screenshots to quick settings was to not lose the app actions

[leolost2605]

Ok the current version of this PR is now a hybrid. If it's interactive we will use the portal if not we still go directly via the gala dbus. This allows us to keep everything while reducing code duplication (the interactive dialog window).

In addition to this I've opened flatpak/xdg-desktop-portal#1415 and elementary/portals#105 which should hopefully let us use the portal for everything in the future. But since this is probably OS 9 material (or will never happen if the portal maintainers disagree) I still think we should go with this for now.

[danirabbit]

One other small interaction problem: when clicking "Cancel" on the save dialog I think we should launch the portal in interactive again since you probably want to redo your screenshot if you cancel here

[danirabbit]

Ah sorry one more thing, I guess we can clean up unused gsettings as well from the schema

[danirabbit]

Really great cleanup. Works totally as expected afaict