Define wxICON_IS_BITMAP when this is indeed the case

This symbol is useful for testing if wxIcon can be just converted to
wxBitmap, as is the case in all ports except for wxMSW currently.

docs/doxygen/mainpages/const_cpp.h

@@ -401,6 +401,9 @@ more details.
         defined when compiling code which uses wxWidgets as a DLL/shared library}
 @itemdef{WXBUILDING,
         defined when building wxWidgets itself, whether as a static or shared library}
+@itemdef{wxICON_IS_BITMAP,
+         defined in the ports where wxIcon inherits from wxBitmap (all but
+         wxMSW currently)}
 @endDefList
 
 */