Add wxTreeCtrl::SelectChildren() method.

Add MSW and generic implementation, documentation and change to the sample
showing it.

Closes #11620.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63277 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/treectlg.h

@@ -155,6 +155,7 @@ public:
     virtual void Unselect();
     virtual void UnselectAll();
     virtual void SelectItem(const wxTreeItemId& item, bool select = true);
+    virtual void SelectChildren(const wxTreeItemId& parent);
 
     virtual void EnsureVisible(const wxTreeItemId& item);
     virtual void ScrollTo(const wxTreeItemId& item);