coremodel: Remove playing state when changing player playlist

When changing the player playlist, the state of the coresongs from the current model are not updated. Thus, the playing state of the current model is unchanged. This means that the play icon of the current model will still be visible once an other model (album, artist, etc) is loaded (the PLAYING state will still be set).

Fix the issue by removing the PLAYING state of the coresong from the current model before changing the player model.