Port menus to Qt 5.

Copy over qtmenu and qtmenuitem from the Qt 4 branch.
Register types in qstyleplugin.cpp.  

Delete Menu.qml MenuItem.qml - no longer in use 
and replaced by C++ code.

Some porting was necessary, in particular changing
QtMenuBase's base class from QObject to QtQuickItem.

Bugs: On OS X the menu bar is displayed only ~50%
if the time. Other platforms are not tested. 
Context menus do not work.

Change-Id: I4e9cc6952ec1c70dc28cfda2352ba9962b539b17
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>