data: Generate dash/app-grid defaults from text files

data: Generate dash/app-grid defaults from text files

Defining default apps as serialized GVariants isn't very human-friendly,
which likely contributes to the fact that the lists are in parts horribly
outdated (Books! Cheese! Screenshot! gedit!).

Instead, generate the lists at build time from simple text files, which
should be much easier to update.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3632>