QTBUG-26296, dock widget moving

QTBUG-26296, dock widget moving

Cocoa lacked implementation of FrameStrut events, and also frameMargins on
QPlatformWindow. Fix both of these issues. Unfortunately QDockWidget also
contains a tangle of #ifdef MAC behaviour which I am unclear about. What's
included here disables some logic on Mac that seems definitely wrong -
while moving a window on Mac we now generate NonClientArea events
(as intended, I believe), but this should not cause dock-widget dragging to end.

Note the window titlebar is the only frame-strut/non-client area on Mac (as far as I can see)

Task-number: QTBUG-26296
Change-Id: Id0c6e954db64b9f9f71d16355cb92922877e5ebe
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>