layout: Implement `text-indent` per CSS 2.1 § 16.1.

I had to use a somewhat unconventional method of computing text
indentation (propagating from blocks down to inlines) because of the way
containing blocks are handled in Servo.

(As a side note, neither Gecko nor WebKit correctly handles percentages
in `text-align`, at least incrementally -- i.e. when the percentages are
relative to the viewport and the viewport is resized.)

r? @SimonSapin