macOS: Key modifiers aren't ignored when dragging 'within the application'

This was broken by QTBUG-99471.

When running the draggableicons example on macOS I can force a move by pressing cmd and force a copy by pressing alt. On Windows and Linux this is not possible (as expected).

Note that the fix for QTBUG-99471 wrongly assumes that sourceOperationMaskForDraggingContext is called for whichever context something is dragged to. In my tests sourceOperationMaskForDraggingContext was always called twice when starting a drag, first with NSDraggingContextWithinApplication and then with NSDraggingContextOutsideApplication.