Expression parsing is awesome

You know how I said that adding anything with overridden operators to the expression parser would be pretty easy?

Actually, it's ridiculously easy.  I just added std::string as a basic type, and it took a whole 5 minutes.

I can use strings in my expression parser... that is equal parts exhilarating and terrifying.  :)

Anyway, I'm going to bind UI controls to C++ code via expressions.  The variable scoping rules mean I can bind an expression to a specific object/component and basically manipulate it directly.

I really need to get function calling working for button events.  I can parse the argument list easily enough, I just haven't figured out exactly how I want to bind it to C++ code yet.