qt: fix media library playlists disappearing in the interface

With the ONLY option, playlists can disappear in the interface when the user adds a different type content, such as by dragging. This is problematic because the interface may not know whether the dragged media is considered as video or audio by media library, so it is not possible to (even if we want) disallow (at least from the interface side) adding different type of media. Not to mention, media library can be manipulated by anything, so such blockage would probably does not make sense.

This is especially important because we currently do not offer a view that shows all or mixed playlists.