[async] add "theme loaded" msg

src/concurrent/msg.h

@@ -1,14 +1,22 @@
 #pragma once
 
+struct msgq;
+
 enum {
     ax_msg_shutdown_TAG,
     ax_msg_load_font_TAG,
+    ax_msg_theme_loaded_TAG,
 };
 
 struct ax_msg_shutdown {
 };
 
 struct ax_msg_load_font {
+    struct msgq* callback_mq;
     struct ax_theme* theme;
     int category;
 };
+
+struct ax_msg_theme_loaded {
+    struct ax_theme* theme;
+};