After the last rounds of redesign, it's time to update
the appdata accordingly:
- update screenshots
- add screenshot captions
- point homepage to apps.gnome.org
- mark as mobile adaptive
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3067>
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
