AGW on Phoenix: force a redraw after `ExpandAllChildren` has been called on `customtreectrl.py`. Back-ported from Classic.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/Phoenix/trunk@75531 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775