Merge branch 'safe-show-message'

Show message box from wxSafeShowMessage() in the other ports too, if
possible.

Currently this is done using wxMessageBox() if it can be sure that it's
safe to call and only MSW uses native function which is always safe to
call. Ideal would be to also use a native function under Mac, where it
should also be the case, but this doesn't seem to work for whichever
reason.

See https://github.com/wxWidgets/wxWidgets/pull/2270

interface/wx/app.h

@@ -859,6 +859,18 @@ public:
     */
     bool GetUseBestVisual() const;
 
+    /**
+        Returns a pointer to the top application window if any.
+
+        This function is safe to call even before creating, or after
+        destroying, the application object, as it simply returns @NULL if it
+        doesn't exist. Otherwise it's equivalent to calling
+        @c wxTheApp->GetTopWindow().
+
+        @since 3.1.5
+     */
+    static wxWindow* GetMainTopWindow();
+
     /**
         Returns a pointer to the top window.