Dialog::title

Dialog is incomplete without built-in support for title. All dialogs in the examples, screenshots, webinars, and blog posts have had a custom title. The Material and Universal designs both have specs for dialog titles. We should provide support for dialog titles, giving the appropriate looks (padding & font) out of the box.

We should get rid of the button-box weirdness, and simply fill in all Dialog styles with a title in the header and button-box in the footer. This is a high-priority task because it will be a behavioral change and needs to be done before Dialog is released in 5.8.