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.

interface/wx/platinfo.h

@@ -138,6 +138,45 @@ struct wxLinuxDistributionInfo
 };
 
 
+/**
+    @class wxPlatformId
+
+    Defines a very broad platform categorization.
+
+    Usually you would use wxPlatformInfo to get all the platform details rather
+    than this class which only distinguishes between MSW, Mac and Unix
+    platforms.
+
+    This class is mostly useful if a short string describing the platform
+    corresponds to the current platform, i.e. the platform under which the
+    executable runs. The recognized strings are:
+
+    - "msw" (preferred) or "win" (for compatibility) for MSW.
+    - "mac" for Apple systems, i.e. macOS and iOS.
+    - "unix" for the (other) Unix-like systems.
+
+    @since 3.1.5
+ */
+class wxPlatformId
+{
+    /**
+        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.
+
+        Returns one of "msw", "mac" or "unix" or an empty string if the current
+        platform is not recognized.
+     */
+    static wxString GetCurrent();
+
+    /**
+        Returns true if the given string matches the current platform.
+     */
+    static bool MatchesCurrent(const wxString& s);
+};
+
+
 /**
     @class wxPlatformInfo
 
@@ -590,4 +629,3 @@ public:
 
     //@}
 };
-