fix(player-mpris-tail): fixes #335

[zjeffer]

This keeps the most recent player in a separate .txt file in the same folder as the Python script.
Executing play/pause/next/prev commands will read that file to make sure the latest active player is the chosen player.

• If one player is playing, show that.
• If multiple players are playing, show the most recently interacted-with player.
• If no players are playing, show the most recently interacted-with player.

Fixes #335. @victorz Can you test this out?