WIP: Accessible Dialog

To make `material-dialog` accessible, focus must be handled to guide assistive tech and keyboard users through the interface. When the dialog opens, focus is sent from the "Open a Dialog" button to the "Okay" / close button. When the dialog closes, focus should be sent back to the original target element (still to be implemented). As part of this change, `tabIndex="-1"` and a `focus()` listener was added to `material-button` so that focus could be dynamically sent to the `innerElement` (a native button or anchor element).