Cocoa: only send move events if we're topmost valid window

QNSView was sending mouse move events through even if there was
another window in our application covering the point where the mouse
was. This change tracks enter and leave events in QNSView to determine
if the move events should actually get sent through to
QWindowSystemInterface

Task-number: QTBUG-30243
Change-Id: Icc3ff5119858fba1e7b5baa210a0663428a4c746