Drag-n-drop improvements

[kenmcgaugh]

This adds support for a visual indicator both of what is being dragged as well as the number of items being dragged (for media items only).

It also fixes some issues with the current system, including stray drag indicators being left behind, runaway scrolling, and inadvertent reordering of media when an item is dropped on itself.

Note that the visual indicator is implemented by changing the system cursor to an image grabbed from the item under the mouse when the drag is initiated. I tried to get this working both by using a Drag attached property as well as instantiating a QDrag object in C++. In both cases it worked visually but the interface stopped receiving drag events and I couldn't figure out how to get that working. Because of this, you still cannot drag anything outside of xSTUDIO which I would like to be able to do in the future.

This has been tested under MacOS only at this point.

[tedwaine]

I've tested on Linux and Windows too, working fine. Nice improvement!

[kenmcgaugh]

Thanks @tedwaine! I still want to get this working using Drag attached properties instead of hijacking the system cursor. Do you have any ideas on why all the drop areas stop receiving drag events? I’ve made sure my mime types are compatible.