InterfaceRef<T> improvements

[BitterSmile]

Hi!
Thank you for this package, I've been using it for a few years in every project!

I've made 2 changes to improve work with InterfaceRef
Firstly, there is new override for Equals. It is added to avoid marking scenes as dirty when it is not needed. Right now in SceneRefAttributeValidator there is

bool valueIsEqual = existingValue != null && 
                                    isCollection ? Enumerable.SequenceEqual((IEnumerable<object>)value, (IEnumerable<object>)existingValue) : value.Equals(existingValue);

so it will always set valueIsEqual to false for InterfaceRef as a new instance of it was created few lines before.

Another change is to handle drag and drop and get proper value. I encountered that problem few times personaly and I see that it was also reported here: #40
It is quite straight forward change