Draft: GSOC 2023: GTK4 Port

Update Pitivi to use GTK4 instead of GTK3

• This MR is the extension of MR-442

Resources:

1. The official porting guide is at https://docs.gtk.org/gtk4/migrating-3to4.html
2. GTK4 official docs : https://docs.gtk.org/gtk4/
3. GTK4 API Reference (Python) : https://lazka.github.io/pgi-docs/#Gtk-4.0

Remaining high priority:

• GtkLayout replacement
• Implement Drag and Drop EventController
• Viewer (Reliant on GtkSink port)
• GtkPopover

Remaining low priority:

• Drop the use of Dialog.run (Partially done)
• Update to GtkFileChooser API changes
• Adapt to drawing model changes (Partially done, remaining for Gtk.Widget)
• Adapt to GtkHeaderBar API changes
• Drop use of GdkScreen
• Replace "configure-event" and "size-allocate" signals
• GdkPixbuf is deemphasized, causing sizing issues and type mismatches (Partially done)
• Symbolic icons colour change issue
• Fix Unittests
• Fix Interactive Intro (Reliant on GtkPopover changes)
• Replace deprecated Gtk.ModelButton