client: Make CmCallback work like other glib callbacks

Instead of passing the user_data as first argument, pass it as last argument like anywhere else. Instead we have the client as first argument as one would expect.

Remove mentions of user_data being a GObject requirement. It's an untyped pointer as user_data elsewhere.

This is a slight API break but it makes things less confusing (at least for me). As I always get stuck on gpointer object and similar things.

examples/simple-client.c

@@ -20,11 +20,11 @@ CmMatrix *matrix;
 CmClient *client;
 
 static void
-simple_account_sync_cb (gpointer   object,
-                        CmClient  *cm_client,
+simple_account_sync_cb (CmClient  *cm_client,
                         CmRoom    *room,
                         GPtrArray *events,
-                        GError    *err)
+                        GError    *err,
+                        gpointer   unused)
 {
   g_message ("\n");