GTK4 will remove the GtkHeaderBar:title property, so stop using it
and set the window's title property instead, as that's what headbars
use in both GTK3 and GTK4 unless explicitly overridden.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1495>
The Extensions app code is now independent enough from the rest of the code
base to move it to its own subprojects, like we did for the extensions-tool.
This allows for stand-alone builds of the app, which we are about to use
for distributing it as flatpak.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1133
