The only reason "gimp" can be used to search for the software (for
instance in GNOME overview) is because it is the Exec (the short name
is written neither in the Name nor Comment sections). When packaged by
systems like flatpak, this key is changed to `flatpak run ...` and
searching "gimp" does not even return the software!
Adding a Keywords entry to our desktop file fixes this.

Apart from "gimp", I take the opportunity to add other relevant keywords
from our website which currently would not display GIMP as a result:
graphic design illustration painting.