Factor out platform detection from XRC code into wxPlatformId

This will allow reusing the same strings in other places.

Also use this as opportunity to add "msw" as the (preferred) synonym for
"win" for general consistency.

src/xrc/xmlres.cpp

@@ -42,6 +42,7 @@
 #include "wx/hashset.h"
 #include "wx/scopedptr.h"
 #include "wx/config.h"
+#include "wx/platinfo.h"
 
 #include <limits.h>
 #include <locale.h>
@@ -585,15 +586,7 @@ static void ProcessPlatformProperty(wxXmlNode *node)
 
             while (tkn.HasMoreTokens())
             {
-                s = tkn.GetNextToken();
-#ifdef __WINDOWS__
-                if (s == wxT("win")) isok = true;
-#endif
-#if defined(__MAC__) || defined(__APPLE__)
-                if (s == wxT("mac")) isok = true;
-#elif defined(__UNIX__)
-                if (s == wxT("unix")) isok = true;
-#endif
+                isok = wxPlatformId::MatchesCurrent(tkn.GetNextToken());
 
                 if (isok)
                     break;