Qt: drag-and-drop: simplification and robustness

Qt: drag-and-drop: simplification and robustness

- Make PlMimeData simply a subclass of QMimeData: destructor of indirect
subclasses of QObject is called even if their direct base class' destructor
is not virtual.
- Protection against using input_item_t* from another VLC instance:
qobject_cast<PlMimeData*>() will fail on QMimeData dropped from another process.
- Some refactoring of PLModel::dropMove() for robustness.