Added wxTreebook:

- added the control itself
- added protected wxBookCtrlBase::AllowNullPage() to accommodate it
- big changes to the sample to get rid of (most) ugly macros
- added XRC handler for the control
- added docs
- and wxUSE_TREEBOOK everywhere


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@35862 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/xrc/xh_all.h

@@ -36,6 +36,7 @@
 #include "wx/xrc/xh_notbk.h"
 #include "wx/xrc/xh_listbk.h"
 #include "wx/xrc/xh_choicbk.h"
+#include "wx/xrc/xh_treebk.h"
 #include "wx/xrc/xh_text.h"
 #include "wx/xrc/xh_listb.h"
 #include "wx/xrc/xh_toolb.h"