Some fairly service agnostic support for "loving" any track.

This commits uses the engine controller (which may not be the best place for it, but not sure where else to put it) to emit a signal when the user "loves" a track.

Interested services (or other parts of the application) can listen for this signal and behave as they wish. I'm bypassing the little action buttons for now as I'm not sure the best way to show only the one, but all the time (if lastfm is active).

This adds a dbus call /Collection loveTrack as well as a global shortcut (default Meta + L) to love the current playing track.