Add a Canvas ID CSS class to the Canvas Element

This patch adds a CSS class in the form `echo-canvas-<canvas-id>` (eg `echo-canvas-js-sdk-canvas-001`) to each canvas as it is initialized. It also provides a `canvasId` property to applications initialised by the canvas - allowing for applications that need to understand the details of the canvas deploying them to do so.

I've included both the change itself and some tests to verify the behaviour.