Add last_focused timestamp to containers

A new field `struct timespec last_focused` is added to the
`sway_container_state` struct holding a UNIX timestamp telling when this
container last gained focus.  This timestamp is added to the IPC `get_tree`
JSON output.

The rationale is that it allows users writing their own container manipulation
scripts (e.g., window switchers) to sort containers by last-recently-used
order.

include/sway/tree/container.h

@@ -52,6 +52,7 @@ struct sway_container_state {
 
 	struct sway_container *focused_inactive_child;
 	bool focused;
+	struct timespec last_focused;
 
 	enum sway_container_border border;
 	int border_thickness;