Review/simplify/cleanup MDI classes for all platforms and introduce base

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

docs/doxygen/mainpages/const_wxusedef.h

@@ -134,6 +134,7 @@ library:
 @itemdef{wxUSE_GAUGE, Use wxGauge class.}
 @itemdef{wxUSE_GENERIC_DRAGIMAGE, Used in wxDragImage sample.}
 @itemdef{wxUSE_GENERIC_DRAWELLIPSE, See comment in wx/dc.h file.}
+@itemdef{wxUSE_GENERIC_MDI_AS_NATIVE, This is not a user-settable symbol, it is only used internally in wx/generic/mdig.h.}
 @itemdef{wxUSE_GEOMETRY, Use common geometry classes}
 @itemdef{wxUSE_GIF, Use GIF wxImageHandler}
 @itemdef{wxUSE_GLCANVAS, Enables OpenGL support.}