Migrate to gtk4 / libadwaita
It's time to start a new migration to the new version of the Gtk library. We need to make use of the new modern Gtk4 lib and libadwaita to support dark theme.
Build with gtk4
Find a replacement for gspell (this could be optional, we can drop the spellcheck support until gspell support gtk4)
Clean the code (remove commented code, debug printf and