Gtk4 bonanza

[Alecaddd]

Summary / How this PR fixes the problem?

Upgrading the dependencies to start using GTK4. This will break the whole UI and more as the API changes are substantial.

If you don't have GTK4 installed in your system, you can't use meson to build but you'll have to rely on flatpak: flatpak-builder build com.github.akiraux.akira.yml --user --force-clean.

Known Issues / Things To Do

• [ ] Figure out a proper replacement/rework of the BaseCanvas that was using GdkWindow, now deprecated in favor of GdkSurface.
• [ ] Upgrade all deprecated API where possible, comment out the most complicated.
• [ ] Make a successful build

This PR fixes/implements the following bugs/features:

• Require gtk4
• Temporarily remove LibGranite as it depends on gtk3
• Update the flatpak to build against GNOME SDK 40
• Build the elementary style and icon

[Alecaddd]

Closing this, deferred it for a later time.