title: Filtering an object field in Unity Editor Extension
body: <p>I'm looking to use Unity's ObjectField to have users search for objects of a particular type. I know the constructor allows for a <code>typeOf(objectType)</code>, but that does not seem to allow for filtering on custom components.</p>
  
  <p>Essentially, a previous editor script I have sets up objects, and places a unique custom script on each of the components, but I've been unsuccessful in utilizing the ObjectField to search for just those objects. If I change the <code>typeOf</code> in my object field to be my component, it is always empty, despite there being many prefabs with that script attached in my project.</p>
  
  <p>Has anyone had any success with this? Using <code>GameObject</code> finds them, but this finds all game objects. Is there any way to restrict this? Or to keep it only looking at particular folders?</p>