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.

swaybar/meson.build

@@ -2,7 +2,6 @@ executable(
 	'swaybar', [
 		'bar.c',
 		'config.c',
-		'event_loop.c',
 		'i3bar.c',
 		'ipc.c',
 		'main.c',