player/Control: don't stop playback in Play()

Instead of stopping playback completely, only CANCEL the queued song
if necessary, and use the SEEK command to play the selected song.
SEEK will take care for current playback state.

src/player/Control.cxx

@@ -55,15 +55,7 @@ PlayerControl::Play(DetachedSong *song)
 	assert(song != nullptr);
 
 	const ScopeLock protect(mutex);
-
-	if (state != PlayerState::STOP)
-		SynchronousCommand(PlayerCommand::STOP);
-
-	assert(next_song == nullptr);
-
-	EnqueueSongLocked(song);
-
-	assert(next_song == nullptr);
+	SeekLocked(song, SongTime::zero(), IgnoreError());
 }
 
 void