Closes#60
Stepping in sliders is a compromise due to the flexibility of the
current slider implementation. I don't want to force types to implement
Add, and I don't like forcing types to require a Step (ie, what's the
appropriate value for f32 to specify as its next value?). Using a
percentage combined with lerp keeps the implementation fairly
straightfoward, although I remember experiencing this type of
configuration in another UI framework a long time ago and thinking it
was a little annoying to work with.
Ultimately, setting actual step boundaries can be done by customizing
the type that the slider is operating over. I feel like that's a much
more powerful design than I've experienced in previous frameworks, so
I'm hoping this percent step behavior is a reasonable compromise.
