classes for wxMDI{Parent,Child}Frame and wxMDIClientWindow.
Also use generic MDI implementation for wxMotif as it seems to be more
functional and definitely is more maintained (we probably should use the
generic version for wxGTK too).
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@56674 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
