Add convenient wxFileName::GetAbsolutePath() wrapper and use it

This wrapper simply combines the calls to MakeAbsolute() and
GetFullPath(), but using it results in shorter and more clear code, so
it seems to be worth having.

include/wx/fswatcher.h

@@ -370,15 +370,7 @@ protected:
 
     static wxString GetCanonicalPath(const wxFileName& path)
     {
-        wxFileName path_copy = wxFileName(path);
-        if ( !path_copy.MakeAbsolute() )
-        {
-            wxFAIL_MSG(wxString::Format(wxASCII_STR("Unable to normalize path '%s'"),
-                                         path.GetFullPath()));
-            return wxEmptyString;
-        }
-
-        return path_copy.GetFullPath();
+        return path.GetAbsolutePath();
     }