Remove support for hiding the menu

Maintaining this feature for Qt 4 and 5 on all platforms has become complex.
Also, removing the menu bar does not result in a better user experience.
From now on, let the window system decide if the bar is displayed in the
global menu, otherwise show it always as in normal applications.
Reduce minimum Qt5 version to 5.2
Fix builds with GCC 4.6 (cmake only)