Port to Gtk4

[danirabbit]

Fixes #205

Todo:

• [x] Fix buttons with icons and labels
• [ ] Verify we didn't break multiple cameras
• [x] Double check error dialog behavior
• [ ] No window with "Failed to register: Timeout was reached" on a computer with a webcam. Maybe something to do with Flatpak permissions?
• [ ] Gstreamer widget is Gtk3, so we need to use a Gtk.Video. Example here: https://gitlab.gnome.org/matthiasc/video-play. Alternatively it looks like there might be a gtk4paintablesink?

[davidmhewitt]

Those errors mean you can't subclass that thing as they're sealed classes. I don't think the necessary annotations are available for it to be a Vala error yet.

[danirabbit]

Ah damn that sucks!

[treppenwitz03]

Managed to make it work with 'gtk4paintablesink' but requires extra setup from installing https://gitlab.freedekstop.org/gstreamer/gst-plugins-rs/ and installing the gtk4 plugin. loading the plugin manually and feeding the paintable to a Gtk.Picture in lieu of Gtk.Widget in CameraView