Simplify transactions by utilising a dirty flag on containers

This PR changes the way we handle transactions to a more simple method.
The new method is to mark containers as dirty from low level code
(eg. arranging, or container_destroy, and eventually seat_set_focus),
then call transaction_commit_dirty which picks up those containers and
runs them through a transaction. The old methods of using transactions
(arrange_and_commit, or creating one manually) are now no longer
possible.

The highest-level code (execute_command and view implementation
handlers) will call transaction_commit_dirty, so most other code just
needs to set containers as dirty. This is done by arranging, but can
also be done by calling container_set_dirty.

sway/commands/split.c

@@ -16,7 +16,7 @@ static struct cmd_results *do_split(int layout) {
 	}
 	struct sway_container *parent = container_split(con, layout);
 	container_create_notify(parent);
-	arrange_and_commit(parent->parent);
+	arrange_windows(parent->parent);
 
 	return cmd_results_new(CMD_SUCCESS, NULL, NULL);
 }