Predefined categories aren't a great way for organizing installed
applications, but they have their use in "stores" like Software
or flathub.
Not listing any category means we fall through the cracks, so
pick the (hopefully) least inappropriate one ...
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1160
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
