Refactor ReactApplicationRoot / DuktapeEngine / UIManager

[nick-thompson]

Right now the ReactApplicationRoot is kind of a monolith of duplicate codepaths that generally breaks down into interacting with and providing an interface for the duktape context, and providing an interface for constructing the View heirarchy. I'd like to split those distinct responsibilities into a DuktapeEngine class and a UIManager class, and let the ReactApplicationRoot be just a simple wrapper composing those two pieces.

This would allow cleaning up, for example, the multiple different ways of registering a native method in Duktape, and the multiple different implementations of marshaling the juce::var to/from duktape.

[nick-thompson]

This also needs complete type conversion from js types to var types. Right now some of the interfacing has it but, for example, the setViewProperty stuff is restricted to the easy cases (bool, number, string). Ideally this all goes through one code path for converting from js->var and back.

[nick-thompson]

Largely addressed in edb738b after the introduction of the EcmascriptEngine. Right now ReactApplicationRoot still does a lot of work that could be refactored into a ViewManager class, but there's some coupling in there that should be carefully looked at during a refactor

[nick-thompson]

This is completed now as of #46 thanks to @JoshMarler