plugin loader: Improve error handling for app install / removal

When app install / removal fails, properly clean up the pending apps
queue on the error path as well. This can happen e.g. after canceling
the polkit authorization dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=727309
https://bugzilla.gnome.org/show_bug.cgi?id=731064
tests: Details page - fixed

The searched app was changed to geary. Also ugly states accesses was
changed to attributes doing the same work.

https://bugzilla.gnome.org/show_bug.cgi?id=725219
Add keywords to the desktop file

Use the same keywords that gpk-application.desktop has, for starters.

https://bugzilla.redhat.com/show_bug.cgi?id=1103472
Updated Turkish translation