Allow plugins to disable themselves

We'll need this for future functionality.
Do not crash if there is no /usr/share/appdata directory

This can happen if no applications with AppData are installed on the system, and
gnome-software is run locally.
Correctly abort if PackageKit transactions fail

Otherwise we'll get critical warnings about re-using GErrors.
Do not install plugins just used in 'make check'
Fix a critical error in gs_string_replace()

A merge conflict was not properly resolved in a previous commit, which left the
call to g_string_erase() with the wrong sign.
Add a module to submit the user rating to the fedora-tagger web service

NOTE: This only works if you are using Fedora as an OS for obvious reasons.