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/loop.h

@@ -0,0 +1,47 @@
+#ifndef _SWAY_LOOP_H
+#define _SWAY_LOOP_H
+#include <stdbool.h>
+
+/**
+ * This is an event loop system designed for sway clients, not sway itself.
+ *
+ * It uses pollfds to block on multiple file descriptors at once, and provides
+ * an easy way to set timers. Typically the Wayland display's fd will be one of
+ * the fds in the loop.
+ */
+
+struct loop;
+
+/**
+ * Create an event loop.
+ */
+struct loop *loop_create(void);
+
+/**
+ * Destroy the event loop (eg. on program termination).
+ */
+void loop_destroy(struct loop *loop);
+
+/**
+ * Poll the event loop. This will block until one of the fds has data.
+ */
+void loop_poll(struct loop *loop);
+
+/**
+ * Add an fd to the loop.
+ */
+struct loop_event *loop_add_fd(struct loop *loop, int fd, short mask,
+		void (*func)(int fd, short mask, void *data), void *data);
+
+/**
+ * Add a timer to the loop.
+ */
+struct loop_event *loop_add_timer(struct loop *loop, int ms,
+		void (*callback)(int fd, short mask, void *data), void *data);
+
+/**
+ * Remove an event from the loop.
+ */
+bool loop_remove_event(struct loop *loop, struct loop_event *event);
+
+#endif