Add "pressed" and "current" wxBitmapButton XRC properties

New names are consistent with the names of the preferred SetBitmapXXX()
functions.

Document them and show using one of them in the sample.

include/wx/xrc/xh_bmpbt.h

@@ -30,7 +30,8 @@ private:
 
     void SetBitmapIfSpecified(wxBitmapButton* button,
                               BitmapSetter setter,
-                              const char* paramName);
+                              const char* paramName,
+                              const char* paramNameAlt = NULL);
 };
 
 #endif // wxUSE_XRC && wxUSE_BMPBUTTON