Choose the right parent

Change-Id: I5a9011b001a712a38e07289355d28f46bddb01b2
Set and use own platform parent window

QWindow::setParent() sets the parent a bit late, meaning we can't
use parent() in any function called from QCocoaWindow::setParent().

Change-Id: I9ac2f08217629ae173d32377b11be948585083f3