Add QPlatformWindow::windowClosestAcceptableGeometry().

Add a function to determine the closest acceptable geometry
(height-for-width) to QPlatformWindow similar to the existing
QPlatformWindow::windowMinimumSize(), windowMaximumSize()...
Clean up the hack in the Windows platform plugin accordingly.

Change-Id: I96e943cfdbafdd0ff444e0d53ee0bdf8603d33df
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>