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.

common/meson.build

@@ -5,6 +5,7 @@ lib_sway_common = static_library(
 		'cairo.c',
 		'ipc-client.c',
 		'log.c',
+		'loop.c',
 		'list.c',
 		'pango.c',
 		'readline.c',