GTK4 blocker bug

Blocking bugs:

• gtk4 GStreamer sink (or Rust "paintable" sink)
• de-clutter-ification
• preferences changes
• properties UI update
• main window code cleanup
• remove libpeas-gtk dep

Benefits:

• UI coherence with other core GNOME apps
• Support for older graphics cards that don't support OpenGL 3.0 (see #523 (closed))