Allow overriding wxMenu::MSWCommand() in wxMSW

This can be useful to implement custom handling of WM_COMMAND for the
popup menus, so make this function virtual and document it to indicate
that it is now part of the public API.

Closes https://github.com/wxWidgets/wxWidgets/pull/2170

include/wx/msw/menu.h

@@ -55,11 +55,12 @@ public:
     // Don't use this object after calling this method.
     WXHMENU MSWDetachHMENU() { WXHMENU m = m_hMenu; m_hMenu = NULL; return m; }
 
+    // Process WM_COMMAND.
+    virtual bool MSWCommand(WXUINT param, WXWORD id);
+
     // implementation only from now on
     // -------------------------------
 
-    bool MSWCommand(WXUINT param, WXWORD id);
-
     // get the native menu handle
     WXHMENU GetHMenu() const { return m_hMenu; }