Implement delayed destruction for wxPopupTransientWindow.

Implement delayed destruction for wxPopupTransientWindow.

Windows of this class can be destroyed at any moment, even while some events
are still being processed, so delay the real destruction until we can be sure
that it's safe to delete the window.

This fixes problems (crashes due to dangling pointers) when the object is
deleted from the overridden OnDismiss(), for example.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69349 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775