QMenuBar: fix extra indent on the right with a TopLeftCorner widget.

When setting a left corner widget on a QMenuBar, the first
action rectangle is offset by its width and thus the width
should not be added to the size hint. Use QSize::expandedTo()
instead.

Task-number: QTBUG-36010
Change-Id: I660e3facbd0aeb5fb84fac8923db3e0c7998309d
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>