dialog should get its buttons from the DOM

The content of a dialog comes from the DOM, and its title comes from an attribute in the DOM, but the buttons and their labels are passed in separate javascript options. This takes some of my content out of my template and puts it in multiple places.

Instead, dialog should look for button elements (perhaps with a particular class), relocate or style them as necessary to match the theme, and provide a convenient method to bind callbacks for them.