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.

include/wx/platinfo.h

@@ -137,6 +137,40 @@ struct wxLinuxDistributionInfo
     { return !(*this == ldi); }
 };
 
+// Platform ID is a very broad platform categorization used in external files
+// (e.g. XRC), so the values here must remain stable and cannot be changed.
+class wxPlatformId
+{
+public:
+    // Returns the preferred current platform name, use MatchesCurrent() to
+    // check if the name is one of the possibly several names corresponding to
+    // the current platform.
+    static wxString GetCurrent()
+    {
+#ifdef __WINDOWS__
+        return wxASCII_STR("msw");
+#elif defined(__APPLE__)
+        return wxASCII_STR("mac");
+#elif defined(__UNIX__)
+        return wxASCII_STR("unix");
+#else
+        return wxString();
+#endif
+    }
+
+    // Returns true if the given string matches the current platform.
+    static bool MatchesCurrent(const wxString& s)
+    {
+        // Under MSW we also support "win" platform name for compatibility with
+        // the existing XRC files using it.
+#ifdef __WINDOWS__
+        if (s == wxASCII_STR("win"))
+            return true;
+#endif // __WINDOWS__
+
+        return s == GetCurrent();
+    }
+};
 
 // ----------------------------------------------------------------------------
 // wxPlatformInfo