player/Control: move IDLE_PLAYER to Player::SeekDecoder()

This emits the event even if PlayerControl::Play() is used to replay
the current song, which emits the missing "player" idle event.

Closes #381

src/player/Thread.cxx

@@ -580,6 +580,8 @@ Player::SeekDecoder()
 {
 	assert(pc.next_song != nullptr);
 
+	idle_add(IDLE_PLAYER);
+
 	pc.outputs.Cancel();
 
 	const SongTime start_time = pc.next_song->GetStartTime();