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/rc/derivdlg.xrc

@@ -27,7 +27,6 @@
             <border>5</border>
             <object class="wxButton" name="my_button">
                 <label>My Button</label>
-                <variant>small</variant>
             </object>
         </object>
         <object class="sizeritem">