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/bookctrl.h

@@ -176,6 +176,13 @@ public:
     }
 
 protected:
+    // Should we accept NULL page pointers in Add/InsertPage()?
+    //
+    // Default is no but derived classes may override it if they can treat NULL
+    // pages in some sensible way (e.g. wxTreebook overrides this to allow
+    // having nodes without any associated page)
+    virtual bool AllowNullPage() const { return false; }
+
     // remove the page and return a pointer to it
     virtual wxWindow *DoRemovePage(size_t page) = 0;