GNOME 40 support

Make the extension compatible with GNOME 40.

• Remove compatibility code for older shell versions
• Port preferences to GTK4
• Test on shell pre-release versions:
  • Night Light timer is working
  • Location timer is working
  • Manual timer is working
  • On-demand timer is working
    • Button is added to the top bar and is working
    • Menu entry is added to the aggregate menu and is working
  • Interface changes are detected
    • GTK theme changes are detected
    • Shell theme changes are detected
    • Icon theme changes are detected
    • Cursor theme changes are detected
    • Wallpaper changes are detected
  • Commands are spawned
  • Error notifications are sent