Add wxApp::GetMainTopWindow() wrapper

This trivial function just allows to avoid checking if wxTheApp is not
null before calling GetTopWindow() on it.

Replace the existing "wxTheApp && wxTheApp->GetTopWindow()" calls with
wxApp::GetMainTopWindow().

No real changes.

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.