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

interface/wx/menu.h

@@ -107,6 +107,16 @@ public:
     */
     void Enable(int id, bool enable);
 
+    /**
+        Returns true if the menu with the given index is enabled.
+
+        @param pos
+            The menu position (0-based)
+
+        @since 2.9.4
+    */
+    bool IsEnabledTop(size_t pos) const;
+
     /**
         Enables or disables a whole menu.