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!


Also I'm not sure whether updating the gui items after every operation is the right approach. It works and is safe for now and I might be prematurely optimizing here, but I think only operations that modify the array (`add`, `delete`, `map!`, `<<`...) should be affected.