MDL-35918 javascript: dialogues screen reader support

MDL-35918 javascript: dialogues screen reader support

The YUI notification dialogues will now set the appropriate
aria attributes when they are shown/hidden to allow screen
readers to navigate them correctly.

When the dialogue is shown all background elements will be marked
as hidden while the dialogue is marked as visible to a screen reader.

When the dialogue is closed the modified elements will be reverted
to their previous state and the dialogue will be marked as hidden to
a screen reader.