Canvas properties

By default any element that is tab-focussable (via tabindex) gets an outline rendered by certain browsers. We turn this outline off for canvas, by setting the canvas's style property "outline: none". We also disable the onmousedown handler and text-select properties so that the canvas does not count as a text-selectable item. Click-dragging the canvas will not show the typical I-bar cursor associated with text selection, and double clicking the canvas will not select it as if it was a text element.