don't show id on anonymous slide (slide w/o title)
enhance and cleanup document template

- use new doctitle API to emit doctitle
- use proper viewport value
- allow theme and transition to be set from query string
- hide internal comments
- use doctitle as title slide unless showtitle attribute is disabled
- cleanup formatting
use <small> tag instead of <p> if paragraph has "small" role
allow background to be set on block image