slide-toggle: refactor to use checkbox as base

[devversion]

The slide-toggle component should use the checkbox component as its base (as discussed in #90).

We planned three steps for the slide-toggle component.

• Basic Skeleton
• Refactor to use Checkbox as base
• Add Dragging functionality (Tracking in slide-toggle: add dragging functionality #600)

[jelbourn]

Some of the code should also be reusable in the forthcoming button-toggle #517

[devversion]

Let's use #610 to track this issue.

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