Add before/after redraw hooks for UI Elements.

[darius98]

While these hooks should rarely be used, it is much safer and cleaner to use them than to override redraw or have side-effects in render.

While afterRedraw hook was discussed, beforeRedraw is added for future-compatibility with the (discussed) possibility of separating this.options and this.state: if options that are passed in from a parent's redraw() would influence the internal state, beforeRedraw() can be used to handle this case.