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.c

@@ -0,0 +1,60 @@
+#include <stdio.h>
+#include <string.h>
+
+#include <SDL.h>
+
+#define DMON_IMPL
+#include "dmon.h"
+
+#include "dirmonitor.h"
+
+static void send_sdl_event(dmon_watch_id watch_id, dmon_action action, const char *filepath) {
+  SDL_Event ev;
+  const int size = strlen(filepath) + 1;
+  /* The string allocated below should be deallocated as soon as the event is
+     treated in the SDL main loop. */
+  char *new_filepath = malloc(size);
+  if (!new_filepath) return;
+  memcpy(new_filepath, filepath, size);
+#ifdef _WIN32
+  for (int i = 0; i < size; i++) {
+    if (new_filepath[i] == '/') {
+      new_filepath[i] = '\\';
+    }
+  }
+#endif
+  SDL_zero(ev);
+  ev.type = SDL_USEREVENT;
+  ev.user.code = ((watch_id.id & 0xffff) << 16) | (action & 0xffff);
+  ev.user.data1 = new_filepath;
+  SDL_PushEvent(&ev);
+}
+
+void dirmonitor_init() {
+  dmon_init();
+  /* In theory we should register our user event but since we
+     have just one type of user event this is not really needed. */
+  /* sdl_dmon_event_type = SDL_RegisterEvents(1); */
+}
+
+void dirmonitor_deinit() {
+  dmon_deinit();
+}
+
+void dirmonitor_watch_callback(dmon_watch_id watch_id, dmon_action action, const char *rootdir,
+  const char *filepath, const char *oldfilepath, void *user)
+{
+  (void) rootdir;
+  (void) user;
+  switch (action) {
+  case DMON_ACTION_MOVE:
+    send_sdl_event(watch_id, DMON_ACTION_DELETE, oldfilepath);
+    send_sdl_event(watch_id, DMON_ACTION_CREATE, filepath);
+    break;
+  case DMON_ACTION_MODIFY:
+    break;
+  default:
+    send_sdl_event(watch_id, action, filepath);
+  }
+}
+