Road to GTK4

[hongquan]

GTK4 has been released for a while and GTK3 will be deprecated. CoBang should migrate to GTK4 before we add more features.

The roadblock is how to rendering video (from webcam) to GTK4 widget. There is no Python binding for gst-plugins-rs which provides Gstreamer Sink for GTK 4. The only way to use this plugin is to rewrite CoBang in Rust language. In Rust, we can also use aperture which simplify usage of GTK4 Sink.