PlaylistController: don't queue tracks on middle-click, just append & play

...if nothing is already playing.

This is the last change to mouse playlist-related behaviour, which
hopefully makes everyone happy. Sorry for all the bumps along the road.

Playlist::AddOptions::StartPlay behaviour was improved, it now tries
harder to start playing the first added track. However, if there is
something in the queue, it just gives up.

Revised ChanegLog entry:

CHANGES:
* Playlist-related actions were harmonized, double-clicking or pressing enter will
append tracks to playlist, using any "play media" action will prepend tracks to queue
and immediately start playing; middle-clicking or will append & play while other
append or replace actions will no longer start playback. (BR 145468, BR 145490,
BR 194549)