Refactor owner-drawing code.

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

include/wx/msw/listbox.h

@@ -152,9 +152,6 @@ protected:
     bool m_updateHorizontalExtent;
     virtual void OnInternalIdle();
 
-    // free memory (common part of Clear() and dtor)
-    void Free();
-
     unsigned int m_noItems;
 
 #if wxUSE_OWNER_DRAWN