New screenshot UI

The new screenshot UI™, which I worked on and posted about over the summer as part of GSoC 2021. Based on this design issue and mock-up.

Requires mutter!1893 (merged) with an additional fix, mutter!1899 (merged) and mutter!1967 (merged). I've got everything needed in this Mutter branch, although there's usually also some unrelated stuff there.

The follow-up MR !2107 (merged) should be merged together with gnome-settings-daemon!267 (merged) and gnome-control-center!1075 (merged).

TODO (screenshot part):

• Saving screenshots into a file (rather than just clipboard)
• New icons from Tobias
• Adding screenshots to recent files (see !1439 (diffs) for an example)
• Asynchronous PNG compression
• Starting selection + way to draw a new selection (e.g. modifier + drag)
• Better resizing (resize like windows instead of the current corner handles)
• Making the panel fade out and not receive input when dragging the selection
• Keyboard navigation
• Removing default PrtSc bind from gnome-settings-daemon and adding it here
• Figuring out how to handle GNOME Shell popups
• Tooltips with shortcuts for panel buttons
• Checkbox on selected window
• Click on notification to open the image
• Fix global shortcut blocking (e.g. mute)
• Add fake search entry to open the UI
• Fix multiple screenshots in 1 second / existing file
• Accessibility review—not sure how to get the screen reader to read nested gnome-shell elements

TODO (screencast part):

• Area and screen recording
• Area indicator during recording
• Fixing GStreamer crashes
• Done notification
• Save to Videos/Screencasts

Future work:

• Use lower scale for area and screen shots when possible on mixed DPI
• Smooth animation to window selection for screenshots
• Window selection for recording
• Desktop audio and microphone audio toggles

COPR for F35 if you'd like to give it a spin: https://copr.fedorainfracloud.org/coprs/yalter/gnome-shell-screenshot-ui/

cc @verdre @bertob @aday