With the previous preparations in place, it is time to take the plunge.
As both the app and the portal use the same small library for handling
external windows, port everything at once to avoid the hassle of building
and installing two versions of the library.
With the portal using GTK4 now, all extensions must port their preference
widgets as well.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1495>
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
