core: watch for configuration file changes

Automatically reset picom and reload the configuration when a change in
the configuration file is detected.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/file_watch.h

@@ -0,0 +1,10 @@
+#pragma once
+#include <stdbool.h>
+
+#include <ev.h>
+
+typedef void (*file_watch_cb_t)(void *);
+
+void *file_watch_init(EV_P);
+bool file_watch_add(void *, const char *, file_watch_cb_t, void *);
+void file_watch_destroy(EV_P_ void *);