Create MediaPlayerBlock using a MPRIS2 library

[DesmondTMB]

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