Migrate to GTK 4 + Libadwaita

[hongquan]

Before adding QR code generation feature, before making UI more complex, let's move on the latest UI technology, so that the job of refactoring UI code less difficult.

Also, let's use Blueprint as language for UI. XML is too verbose. When using GTK 4 and Blueprint, we have to have a step to build UI files, so the current approach of running Python app in development have to be changed, we cannot run directly in-develop source file anymore. So how the new project structure should be? We can take https://gitlab.gnome.org/GabMus/swatch as reference.

[hongquan]

This work is blocked by the lack of a GStreamer + GTK4 plugin.

[hongquan]

https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs