Only keep common code in the base class and extract all menu/listbox-specific
stuff into derived classes.
This makes the code cleaner and more maintainable but introduces some problems
in wxCheckListBox appearance which will be fixed by the next patch.
Closes#10635.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63220 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
