Make translatable the copyright in about dialog

Author: rbuj

src/callbacks.cpp

@@ -186,9 +186,9 @@ cb_about (GtkAction *action, gpointer data)
         "name",               _("System Monitor"),
         "comments",           _("View current processes and monitor system state"),
         "version",            VERSION,
-        "copyright",          "Copyright \xc2\xa9 2001-2004 Kevin Vandersloot\n"
-                              "Copyright \xc2\xa9 2005-2007 Benoît Dejean\n"
-                              "Copyright \xc2\xa9 2011-2018 MATE developers",
+        "copyright",          _("Copyright \xc2\xa9 2001-2004 Kevin Vandersloot\n"
+                                "Copyright \xc2\xa9 2005-2007 Benoît Dejean\n"
+                                "Copyright \xc2\xa9 2011-2019 MATE developers"),
         "logo-icon-name",     "utilities-system-monitor",
         "authors",            authors,
         "artists",            artists,