Add the Keywords entry to the .desktop files

The documentation is rather scarce on what kinds of keywords one is supposed to
put in there, so I took a shameless look at what the LibreOffice folks are doing
[1] and played a copycat.

[1] http://cgit.freedesktop.org/libreoffice/core/commit/?id=9d844ce3f8e411bfb4237372b1a6d4aec21a0a65

BUG: 332641
REVIEW: 117191