Using a ProxyArray to get ListBox items handled correctly

Meant to not make a new branch, but I accidentally screwed up the old one when trying to revert it and decided to delete it. (See #992).

Anyways, to repeat: this intends to address #820.

This time my approach is an ArrayProxy that inherits from BasicObject and delegates everything to Array. 

Please let me know if this approach is good, needs tweaking, or needs a totally new direction (for example, how about `class ProxyArray < Array`?).

I have some questions / comments. I'll put those in-line before :poultry_leg: :sweet_potato: :fork_and_knife:  time!


I think the test problems I was having were from killing the response from the send. So the tests like [this](https://github.com/shoes/shoes4/blob/master/shoes-core/spec/shoes/list_box_spec.rb#L35-L37) were failing because the Array and ProxyArray aren't equal. But I'm inferring (and will investigate shortly) that if I handle the `method_missing` / `send` stuff correctly this will go away.