QMenu::addSection on OS X: OS doesn't support texted separators

Qt5 gained QMenu::addSection() (maybe from KMenu::addTitle()), but this function does not work properly on Mac OS X.
It tries to make an underlined menu item (OS X jargon for a menu action) by creating a QAction with the specified text (and icon), and then adding a separator to it using `action->setSeparator(true)`.

On OS X, this turns the action into a separator, which IMHO is worse than creating a visually non-distinct item.

For the KDE4 equivalent (KMenu::addTitle()), I patched the code so that on OS X it adds both the text item and 1 or 2 separators above and/or below the title text item depending on the location in the menu; I deactivate the title text item to make it visually distinct. Other solutions are of course possible.