Switch to GTK4

From @slomo

https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/blob/main/video/gtk4/examples/gtksink.rs#L23-60 (for the equivalent of the gtkglsink / gtksink switching code, but actually checking if there's GL support instead of just checking element existence), and https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/blob/main/video/gtk4/examples/gtksink.rs#L72 for putting it into a gtk::Image for widget purposes