Add a more complete example of different controls variants in XRC.

Add a dialog showing all the existing variants of a few common controls to
allow seeing at a glance how do they look.

See #16247.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77002 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/xrc/myframe.h

@@ -49,6 +49,7 @@ private:
     void OnPlatformPropertyToolOrMenuCommand(wxCommandEvent& event);
     void OnArtProviderToolOrMenuCommand(wxCommandEvent& event);
     void OnVariableExpansionToolOrMenuCommand(wxCommandEvent& event);
+    void OnVariants(wxCommandEvent& event);
     void OnRecursiveLoad(wxCommandEvent& event);
     void OnAnimationCtrlPlay(wxCommandEvent& event);