Add CheckBox, Switch and ToggleButton widgets

 * Adds CheckBox, Switch and ToggleButton widgets.
 * small update to Button to remove showLabel property (issue #103) (does not address any Button theming issues)

This PR is for getting initial reviews, in particular on:
 * use of FormValueWidget vs FormWidget ?
 * should ToggleButton be a HTMLButtonElement (<button is="d-toggle-button"> or a HTMLElement wrapping an input) ? I chose the former.
 * ToggleButton inheritance
 * focus management

Missing:
 * ToggleButton functional tests
 * Switch bidi support
 * bidi support in general
 * ToggleButton holodark and ios themes

Known issues:
 * Switch interaction is broken when wrapped inside a ```<label>```