We finally have everything in place for distributing the Extensions app
as flatpak without jumping through too many hoops. Add a manifest that
can produce a nightly build like other GNOME modules, and can serve as
a template for a stable manifest on flathub.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1133
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
