Bug 32220: Improve the letterboxing experience

CSS and JS changes to alter the UX surrounding letterboxing. The
browser element containing page content is now anchored to the bottom
of the toolbar, and the remaining letterbox margin is the same color
as the firefox chrome. The letterbox margin and border are tied to
the currently selected theme.

Also adds a 'needsLetterbox' property to tabbrowser.xml to fix a race
condition present when using the 'isEmpty' property. Using 'isEmpty'
as a proxy for 'needsLetterbox' resulted in over-zealous/unnecessary
letterboxing of about:blank tabs.