Add API to detect if a drag and drop operation is active

It would be nice if it was possible to query if a drag and drop operation is currently active, similar to QApplication::activeModalWidget() and QApplication::activePopupWidget().

This would make it easier to control code execution while a drag and drop operation is active, especially if the programmer does not control the execution of QDrag (for example if they are using the convenience drag and drop functionality in Qt).

This is particularly true on Windows, where QDrag::exec() blocks Qt's event loop, but still processEvents() is frequently called, which would deliver timer events (and other events) which in turn could block the drag operation!