Implement wxMenuBar::EnableTop() for wxMSW and wxGTK.

This method was only available in wxUniv before and just always returned true
in the other ports. Implement it for wxMSW and wxGTK too now and document it.
Also add a unit test.

Closes #795.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70350 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/menu.h

@@ -33,6 +33,7 @@ public:
     virtual wxMenuItem* FindItem( int id, wxMenu **menu = NULL ) const;
 
     virtual void EnableTop( size_t pos, bool flag );
+    virtual bool IsEnabledTop(size_t pos) const;
     virtual void SetMenuLabel( size_t pos, const wxString& label );
     virtual wxString GetMenuLabel( size_t pos ) const;