propagate size hints means that we should forward minimum/maximum
size set on QWindow to the underlying native window to restrict
how the user can resize the window. On iOS this does not make
sense, but nevertheless, if we don't override the function, the
default implementation will issue a warning. This again will
always make creator inform that the application ended with error
upon exit.

Change-Id: I0a8bd74c47fafe2115add5b6eb4e77616fcbc365
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>