Create MediaPlayerBlock using a MPRIS2 library#132

I think it might be beneficial to rewrite the MediaPlayerBlock based on a MPRIS2 library to more easily add features to it. Features I have in mind are: - everything that is possible already - automatically selecting an instance, when one is not explicitly given - automatically selecting a player when one is not explicitly given (which makes it possible to have just one block for all players) - Adding a `short_text_format` - controlling the player through click events: - play/pause - next media - cycle through eligible players - toggling between normal text or always short text - an option to make the block disappear from the bar when no player is