Add built-in support for menu in headerview

The windows explorer has the possibility to do filtering by opening a menu in the header of each column:

QHeaderView should support this too.
A very simple proof of concept is attached: headerview.diff

Things to be done:

• add two new QStyle::ComplexControl enumerations for subControlRect/hitTestComplexControl to be able to distinguish between clicking on the header and clicking on the dropdown in the header
• modify QStyleOptionHeader to notify about if there is a menu indicator or not
• adopt QStyle::drawControl() accordingly