overview: Avoid featuring same category twice

Don't show apps from the same category twice, first in the "Editor's
Picks" section and then in the "Recommended <category>" section as well.
It can look weird if we have a bunch of games in the "Recommended Games"
section, and then "Editor's Picks" end up being all games too.
Update help strings for command line arguments

https://bugzilla.gnome.org/show_bug.cgi?id=735023
trivial: Bump plugin version after recent plugin API changes
trivial: Fix make check