themes: rename theme "Default" into "System".

Since we have many themes now, this new name better indicates that it
is meant to follow your desktop theme settings.
Also it will likely not remain the default theme.

libgimpbase/gimpenv.c

@@ -814,7 +814,7 @@ gimp_gtkrc (void)
 
   if (! gimp_gtkrc_filename)
     gimp_gtkrc_filename = g_build_filename (gimp_data_directory (),
-                                            "themes", "Default", "gtkrc",
+                                            "themes", "System", "gtkrc",
                                             NULL);
 
   return gimp_gtkrc_filename;