Menu: Factor MenuContentItem out

Preliminary patch to get non-windowed menus on Android, iOS, etc.

Change-Id: Idc17009baa3a647ec2fbc353f1360fb9ca9e051a
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Menu: Extract QQuickMenuPopupWindow core popup behavior

... and put it in QQuickPopupWindow.

Change-Id: I6920f5c13904483310faae990c7199db9f1c4708
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>