Provide an application-level method of controlling the animation

[vanvoljg]

Right now, there are no friendly ways to control the animation. There's no standardized means of telling the application to play the next/previous animation, or to change the animation's configuration.

This is especially apparent in lib/xebow_web/live/matrix_live.ex on L71, where there's logic for switching animations duplicated between the live view module and lib/xebow/keyboard.ex.

We'll need to figure out a way to do this. I'm pretty sure it'll involve some refactoring, and will likely touch a couple other issues.