TPAC: Centralize font size definitions

TPAC: Centralize font size definitions

The TPAC CSS had a mix of absolute and proportional font size
definitions, making it impossible to define a single base size and have
the layout adjust appropriately, as well as making it difficult to tweak
sizes to, say, eliminate the distinction between 11px and 12px text.

Accordingly, create a separate TT2 file to define standard font sizes,
define one base font size, and then use proportional percentage-based
size definitions for a set of standard font sizes. This way, a site
could easily adjust the base font size to 15px and the rest of the
layout should automatically adjust, and/or revert to using absolute font
sizes for different categories if they prefer.

Note that I did simplify some of the categories of font sizes, as the
distinction between 111% and 110% was probably not evident to most
humans at most font sizes, and even the distinction between 120% and
125% would be 14.4px vs. 15px given a base size of 12px - which
different user agents could render differently anyway.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Ben Shum <bshum@biblio.org>