Dialog: optimize initialization

Assorted changes to speed up loading:

• _create: Delay appending the container div to the body until the end of the function. Since more manipulations are going to occur between the creation of that div and the end of that function, it's adding work for the browser to append it at an early stage.
• open: There's currently a chain of concat calls to find the first tabbable element. Since the first query may return a match, it's wasteful to keep looking if there was a match.
• _createButtons: A couple divs are created, but they're not necessarily used. Check whether there are any buttons before doing the work. Returning any resulting div facilitates "subclassing" this plugin.