Implement directory monitoring using septag/dmon

Use a notification based directory monitoring based on the
septag/dmon lirbary instead of periodically rescan the whole
project's tree.

src/dirmonitor.h

@@ -0,0 +1,14 @@
+#ifndef DIRMONITOR_H
+#define DIRMONITOR_H
+
+#include <stdint.h>
+
+#include "dmon.h"
+
+void dirmonitor_init();
+void dirmonitor_deinit();
+void dirmonitor_watch_callback(dmon_watch_id watch_id, dmon_action action, const char *rootdir,
+  const char *filepath, const char *oldfilepath, void *user);
+
+#endif
+