Support multi-mouse DnD: get rid of QDragManager

Low priority because from a requirements perspective, it's probably the last thing that needs to be done after we support multiple mice in every other way.  Anyway... QDragManager has a suspicious name, and it's a singleton.  So it's another obstacle to supporting multiple input devices.  But it gets used in QApplication, src/plugins/platforms/windows/qwindowsdrag.cpp src/plugins/platforms/xcb/qxcbdrag.cpp src/gui/kernel/qplatformdrag.cpp qevent.cpp etc. so that's a multi-platform change to get rid of it.

If any platforms support multi-mouse DnD (and maybe even if they don't), it probably makes sense to move the state that this stores into QPointingDevicePrivate, or EventPointData.  The device objects are just as available on multiple layers.

QDragManager::drag() has a little extra functionality besides storing state, but not much.