1. added wxTreeCtrl::DeleteChildren()

2. wxTreeCtrl::CollapseAndReset() doesn't destroy the item itself any more
3. wxTreeCtrl::Delete() sends wxEVT_COMMAND_TREE_CHILDREN event (but
   DeleteChildren() and DeleteAll() do not!)
4. Sample modified to show the new function too
5. Micro redraw bug fixed (small vertical line was sometimes drawn when it
   shouldn't have been)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@1224 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775