add SetNativeTheme() (slightly modified patch 1884553)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@52149 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/app.h

@@ -36,6 +36,7 @@ public:
      * (palette/visual/colorcube). under wxMSW, OnInitGui() does nothing by
      * default. when overriding this method, the code in it is likely to be
      * platform dependent, otherwise use OnInit(). */
+    virtual bool SetNativeTheme(const wxString& theme);
     virtual bool OnInitGui();
 
     // override base class (pure) virtuals