D8350: Add categories in gallery desktop file

[Aleix Pol Gonzalez]

apol accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R169 Kirigami

REVISION DETAIL
  https://phabricator.kde.org/D8350

To: arojas, mart, apol
Cc: ngraham, plasma-devel, apol, davidedmundson, mart, hein