WIP: Accessible Dialog

[marcysutton]

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).

[ajoslin]

Merged via 8f756fc, with a few changes:

• If no element with class dialog-close is found, it will focus the last button within the .dialog-actions element.
• Added a note to the documentation saying this.
• Added one unit test for each case: it should focus the .dialog-close element and it should focus the last button.
• Added a TestUtil.mockElementFocus() function, which mocks angular.element.prototype.focus for the duration of one test to set $document.activeElement, so we can actually figure out what got focused in a unit test.

[marcysutton]

I think we need to revisit this–focus is not returned to the triggering element when the dialog is closed.