refactor: use proper dependency tracking

As you can see, there is quite a tangled web of dependency
in this project, which is something to look at later.

Fix #33

src/class/dashboard/init.php

@@ -7,6 +7,9 @@
  * @since  1.5.0
  */
 
+require_once dirname( __FILE__, 2) . '/settings.php';
+require_once dirname( __FILE__ ) . '/subpage-main.php';
+
 /**
  * Handles the Settings interface of the Plugin.
  *