qt: enable drop support in playlist media list view

With this patch, it is now possible to drag media library and input items directly into a playlist.

New input item is created for an unknown source. So, actions such as dragging from file browser should also be supported as long as the drag event contains URLs.

It is also possible to append a playlist into another playlist.

Previously, it was only possible to first load the playlist in order to add items. Now, in the playlist view (the view where all the playlists are listed) items can be dropped in to the playlists.