Extract wxLOCALE_XXX constants into their own header

No real changes, just make it possible to use these constants without
including the entire wx/intl.h.

Note that the existing headers including wx/intl.h need to keep
including it, unfortunately, as not doing it any longer would break
compilation of any code including them which relies on _() or
wxGetTranslation() being defined after including e.g. wx/dc.h. This was,
of course, never guaranteed, but in practice it worked for a very long
time, so don't break it now.

This commit is best viewed with --color-moved git option.

build/cmake/files.cmake

@@ -651,6 +651,7 @@ set(BASE_CMN_HDR
     wx/fswatcher.h
     wx/generic/fswatcher.h
     wx/lzmastream.h
+    wx/localedefs.h
 )
 
 set(NET_UNIX_SRC