Add order functionality for anchors buttons

[KaliedaRik]

Issue

We need to find a way to allow anchors and buttons to have their tab ordering set dynamically.

• We cannot use the tabindex attribute on the hidden <a> and <button> elements as we have no way of knowing how such changes would interfere with the tab ordering in the web page beyond the canvas element.
• So we make a guarantee to devs that all hidden <a> and <button> elements associated with the canvas will always have a tabindex value of 0.
• Instead, we can add an order attribute to the SC Anchor and Button objects. Then, when objects are added/removed, or the order value of existing object changes, we can rebuild the Canvas wrapper's hidden <nav> element so that the elements are listed in the correct tabbing order.

[KaliedaRik]

We can also make a case for 'disabling' a link. Example use case: the entity defining the link gets hidden or moved out of view, in which case the <a> link should also disappear for users who choose to navigate via keyboard tabbing.

• Doable by adding a disabled attribute to the Anchor object - when true, the <a> element is removed from the hidden <nav> element; when false, it gets added back in.