The purpose is making accessing the current playlist entry saner when
commands are executed during initialization, termination, or after
playlist navigation commands.
For example, the "playlist_remove current" command will invalidate
playlist->current - but some things still access the playlist entry even
on uninit. Until now, checking stop_play implicitly took care of it, so
it worked, but it was still messy.
Introduce the mpctx->playing field, which points to the current playlist
entry, even if the entry was removed and/or the playlist's current entry
was moved (e.g. due to playlist navigation).