extensions-app: Add category in .desktop file

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


(cherry picked from commit 360f5b16)