Paper toggle button with thick track

The "new" toggle button with a thicker track. It hasn't been published yet, but I'd like to get some feedback now. Specifically, @azakus how does <core-a11y-keys> work (can't get this or <paper-radio-button> to toggle with the spacebar)?

With <paper-radio-button> removed from this element, <paper-radio-button> can now be changed so that it matches the spec (with the spacing between the border and center circle; see https://www.google.com/design/spec/components/switches.html#switches-radio-button)

Assigning to @frankiefu since he seems to have put a lot of work to this element.
Done. Also added some CSS classes to allow styling the bar and the button in one rule as discussed offline (see docs).