Let the UI loop die when paused

No reason to refresh the UI if nothing is happening. It's a waste of CPU resources.

Checks if Pithos is playing in "self.update_song_row" returns "self.playing" if it is and "False" if it's not. Returning False lets the  "GLib.timeout_add_seconds(1, self.update_song_row)" in "self.play" die. Pressing play to continue the song restarts the UI loop.