Sort dock clients by class and instance

This is similar to #3820 but does not use qsort but an insertion sort in
con_attach.

Since each bar block automatically gets its own incremental bar id,
bards end up being sorted according to their definition order in the
config file.

For i3bar, the WM_CLASS is modified to include an instance name which
depends on the bar_id. This could be useful for other reason, e.g. users
targeting a specific bar instance.

Fixes #3491

src/handlers.c

@@ -1147,9 +1147,7 @@ static bool handle_strut_partial_change(Con *con, xcb_get_property_reply_t *prop
 
     /* attach the dock to the dock area */
     con_detach(con);
-    con->parent = dockarea;
-    TAILQ_INSERT_HEAD(&(dockarea->focus_head), con, focused);
-    TAILQ_INSERT_HEAD(&(dockarea->nodes_head), con, nodes);
+    con_attach(con, dockarea, true);
 
     tree_render();