Investigate Popover API

[KaliedaRik]

From initial investigations, it's clear that canvas elements can be included in Popover elements. Which means that we need to do work to make sure SC plays nicely with such canvases.

This PR:

• adds a new demo - dom-021 - to investigate Popup functionality
• Fix canvas constructor so that it will pick up title and role attributes set on the HTML <canvas> element, and also allow users to define accessibility label and description text on the canvas by setting new data-label and data-description attributes on the <canvas> element.
• fixes some subtle bugs discovered in the mixin/anchor.js and factory/anchor.js files
• adds a new Button object that can be used by artefacts and entitys. This operates in a similar fashion to the Anchor object, creating and manipulating elements in the DOM which can be invoked by code and tracked when users navigate the page using the keyboard tab page
• Updates across the codebase to use the new Button object. This include a fix which was generating errors where certain entity types were not checking for updates to Anchor objects, leading to a confusing navigation experience eg in demo canvas-027 (the interactive video demo)
• Remove the functionality around scrawlNavigationHold - a <div> element SC was (until now) adding to the top of every web page on which it ran. Totally unnecessary and redundant!
• Update demo canvas-027 to make the action labels (play/pause, listen/mute) tab-navigable