Feature Request: TypeDropdown - Allow filtering by type

[Danny-vD]

Right now one can only filter the type dropdown by assembly name, it would be awesome if we could also filter it by a type.

For example:

using UnityEngine;
using EditorAttributes;

public class AttributesExample : MonoBehaviour
{
        [SerializeField, TypeDropdown(typeof(MyBaseType))] private string typePath;
}

Which would then only show the types of that type and inherited types.

---

This is especially useful when writing scripts that only want a certain type.
This is already possible in the current implementation by checking the returned type yourself, but if this could be enforced by the dropdown that would be a lot more convenient.

It would also be clearer to other team members what possible values should be assigned in the inspector.

[Danny-vD]

I opened a pull-request to add this feature.
See #67