Native applications do not use custom fonts in menus, so we should
avoid it - our own example app 'menus' demonstrates the problem.
To quote Gabriel: Although the Mac HIG don't say that you shouldn't, and there's
even a API to do it (which we're obviously not using properly), the truth
is that nobody does it. Simply put, it looks wrong on Mac (although it's
arguable whether it looks good or bad regardless of the platform).

Task-number: QTBUG-29654
Change-Id: Iffd08ad63d419164102b2e738cdf1ebda1967a05
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>