Move swaybar's event loop to common directory and refactor

* The loop functions are now prefixed with `loop_`.
* It is now easy to add timers to the loop.
* Timers are implemented using pollfd and timerfd, rather than manually
checking them when any other event happens to arrive.

include/swaybar/status_line.h

@@ -14,6 +14,8 @@ enum status_protocol {
 };
 
 struct status_line {
+	struct swaybar *bar;
+
 	pid_t pid;
 	int read_fd, write_fd;
 	FILE *read, *write;