Allow marking wxTreeBook nodes to expand initially in XRC.

Add new "expanded" attribute for XRC nodes of treebookpage class.

Also update the sample and the XRC format documentation.

Closes #13355.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68318 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/doxygen/overviews/xrc_format.h

@@ -1730,6 +1730,9 @@ pseudo-class (similarly to @ref xrc_wxnotebook "wxNotebook" and its
     into the image list.}
 @row3col{selected, @ref overview_xrcformat_type_bool,
      Is the page selected initially (only one page can be selected; default: 0)?}
+@row3col{expanded, @ref overview_xrcformat_type_bool,
+    If set to 1, the page is initially expanded. By default all pages are
+    initially collapsed.}
 @endTable
 
 Each @c treebookpage has exactly one non-toplevel window as its child.