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

docs/changes.txt

@@ -11,6 +11,7 @@ All:
 
 All (GUI):
 
+- added wxTreeBook
 - added wxDialog::SetEscapeId()
 - wxItemContainerImmutable::FindString unified (affects wxRadioBox, wxListBox,
   wxComboBox and wxChoice)