Refactor ToggleWidget, add ToggleButtonWidget, ButtonGroupWidget

* Rename ToggleWidget to ToggleSwitchWidget
* Move the boolean value API to a mixing called ToggleWidget
* Implement ToggleButtonWidget
* Implement ButtonGroupWidget

Also:
* Changed default button style to have a darker fade. It now fades
  from #fff to #ddd instead of #fff to #f0f0f0.
* Fix title of test suite (follows-up 299121e).
* Make structure of demo html consistent with other static html files.

Change-Id: I1bc2faf54c24c23aee5d1db95ab7499b6febaead