fixed jslib to use callbacks, cleaned up

[prostolyubo]

Previously: method names were passed as strings, a SendMessage was used that searches for a monobehaviour with a given name (not quite reliable/efficient way of passing things in Unity).
Currently: methods are marshalled as proper callbacks. A MonoBehaviour in the scene is no longer necessary for this awesome system to work.

I also added small fixes to jslib so that things can be passed around between functions without using window. I suppose it can be further optimized, but I don't know how it ended up so complicated in the first place so I left it generally intact.

[greggman]

Thanks for this. My guess is the reason it was the way it was is that's the way I found in the docs in 2014 from my first Unity<->JS stuff. Thanks to your example I've learned better ways