This virtual dtor is not really needed as manager objects are static
singletons and are not destroyed via their pointer, but add a virtual dtor
just to avoid g++ warnings about its absence.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75666 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775