Keep track of multiple players

Currently, when starting another video, media controls are transfered to the new player.
If the player goes away (tab closed, navigate away, etc) media controls are disabled.
This patch makes it keep track of all players that were playing and transfers control to the previous player instead.
Also have "stop" do the same if there's another player available.

Differential Revision: https://phabricator.kde.org/D14221