qt: drag and drop accessory support in KeyNavigableListView

Move most of the DnD stuff from PlaylistListView to KeyNavigableListView. In the future, KeyNavigableListView should have a more generic name.

If needed, I can add a new file such as DnDListView instead of adding the new stuff directly into the generic list view (I do not have a strong opinion on this at the moment). However, these accessory stuffs should not really cause any overhead. The footer item is not instantiated, and the functionality related to DnD is only enabled when certain properties are set.