Improved material-dialog accessibility

To be accessible, `<material-dialog>` needs additional focus management and ARIA attributes. With this change, an `aria-label` names the element for assistive technologies; in the absence of this attribute, a portion of the dialog's body text is appended as a fallback value. Secondly, after the dialog is closed, focus is now sent back to the triggering element: the click event's `currentTarget`.
@ThomasBurleson this could be improved...but the general idea is to grab a few words of the dialog's body text in the absence of an `aria-label`. I'd love to see this functionality added as a string utility that pulls out an arbitrary number of words.