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/reload.c

@@ -12,6 +12,6 @@ struct cmd_results *cmd_reload(int argc, char **argv) {
 	}
 
 	load_swaybars();
-	arrange_and_commit(&root_container);
+	arrange_windows(&root_container);
 	return cmd_results_new(CMD_SUCCESS, NULL, NULL);
 }