macosx: implement simpler and generic drop view for all drag / drop operations

macosx: implement simpler and generic drop view for all drag / drop operations

- Use the new VLCDragDropView for both CAS and main window drop zone.
  This implements a drop highlight border the same way as in playlist view.
- Remove Brushed Metal Image view and its own drop implementation.
  When having a image view inside a drop zone, you need to use
  VLCDropDisabledImageView which disables the own drop handling of NSImageView.
- Removed more unneeded drop implementations for subviews from CAS