upnp: ask if user wishes to stop playback on remote device on stop

This allow user to detach from a remote upnp player without
stopping playback.
upnp: split attach and play function in player

This avoids delay when attaching to stopped player.
upnp: reindent after previous commit
Merge pull request #4020 from elupus/upnpplayer

UPnP player disconnect/attach improvements