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/msw/wx_base.vcxproj

@@ -823,6 +823,7 @@
     <ClInclude Include="..\..\include\wx\secretstore.h" />
     <ClInclude Include="..\..\include\wx\evtloopsrc.h" />
     <ClInclude Include="..\..\include\wx\lzmastream.h" />
+    <ClInclude Include="..\..\include\wx\localedefs.h" />
   </ItemGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">