Controlled MathField

[ShaMan123]

Related #2

React has 2 approaches for interacting with elements:

1. controlled - value handled in state container and passed to element.
2. uncontrolled - value handled in element internally

I've implemented a hack using to proxy change events (which are the basics of the controlled approach) to react.
It seems to work well.
I suspect there's a bug related in mathlive core. Will open a dedicated one.