Add Static method Qt::DropAction QDropEvent::proposedAction(Qt::KeyboardModifiers modifiers, Qt::DropActions actions)

Add Static method Qt::DropAction QDropEvent::proposedAction(Qt::KeyboardModifiers modifiers, Qt::DropActions actions).

With such a static method, a user may use Qt's heuristic to decide which category of action to implement, even when not using Qt's drag and drop events mechanism.