Our modalboxes have been around basically forever, but we've always sort

of had a half-baked implementation in Djblets. It didn't have any

centralized styling, and oddly, the extensions stylesheet actually

defined the only copy of any styles for modalboxes.
It also just hasn't aged well, with some old styles that weren't

optimal, and absolutely no clear accessibility support.
This change introduces a new stylesheet for modalboxes, and a new

djblets-ui Pipeline bundle that replaces djblets-widgets and

incorporates the modalbox styleesheet. (djblets-widgets was introduced

in 2.0 dev and has yet to ship.)
It also fixes up our accessibility issues. The modalbox and its

components now have appropriate ARIA attributes, and focus is capped to

the modal box. Some ordering issues with element construction have also

been added to make sure we're preparing elements before adding them to

the DOM.
What this doesn't do is move to our new CSS component standard. That

will probably not happen for Djblets 2.0.

Tested with some modalboxes (including the Review Dialog) in Review Board,

removing Review Board's own styling first (and adding the new bundle).

Verified things looked right.
Manually checked the ARIA attributes on the elements.
Tested keyboard focus to make sure I couldn't focus anything else in

the document.