topwin: use postorder traversal in _rtgui_topwin_draw_tree
topwin: show from the activated window in rtgui_topwin_activate_topwin

When topwin A is going to be activate, _draw_tree from A instead of the
root of the topwin. As the parents of A should have already been shown.

Thanks for limxuzheng and hexlog for bug reporting.

Github-hook: fix #39