This makes drag-and-drop easier, because drag-and-drop pixmap does not
cover the area under mouse cursor.

- 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.

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

A helper object keeps reference count until drag-and-drop is finished.

This enforces the use of qtu() or whatever appropriate.

- Use item ids instead of pointers for drag data to prevent usage after freeing.
- The list of selected items that contitutes drag data is sorted correctly according to item position in tree. This keeps their relative order after drop operation even if they originated from different parent nodes.
- If a parent and any of it's children are selected, only the parent enters drag'n'drop operation. Children are thus not moved out of the parent when dropping.
- Copying by drag and drop performs a recursive copy of all children.

Thanks to Srikanth Raju <srikiraju@gmail.com> for pointing out the problem.

...as the FAQ on http://libsdl.org/ declares this is the most portable
method.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

There may be similar bugs in other places

Only blank discs seem to give ID_CDROM_MEDIA_STATE=blank normal audio-cds just
dont have that property at all. And instead defaulting to file, default to
invalid and return NULL if we don't find any property to tell use that optical
disc is something we are able to play currently.

Fixes udev discs sd for me, tested with empty dvd-r discs and audio cds.