[SwitchBase] Add a transition between checked and unchecked icons

[calebissharp]

Uses the Fade component in SwitchBase to transition between checked and unchecked icons. This animation is disabled by default for Switch, as it really only applies to Radio and Checkbox.

Resolves #12639.

[eps1lon]

From looking at the spec it looks like

• the checkmark in Checkbox is animated from left to right
• the circle in Radio grows outwards
• the handle in Switch translates horizontal (which it already does)

I would prefer the right (or at least close to) transition instead of just any.

[mbrookes]

@calebissharp Thanks for working on this, it was a great first pass, but as it's been a while without progress, lets close for now. You're more than welcome to open a new PR an time!