tracking-shorten: use 'shorten-strings' from shorten.el

shorten.el is a new string shortening library which provides
component-wise string shortening, which, by breaking strings into parts,
and shortening the parts in a tree-walk algorithm, can often shorten
strings more than the simple prefix shortener it replaces.  It can also be
configured to shorten better than a simple prefix shortener in every case
by dropping unnecessary tail-position parts, as well as many other
possibilities.

This addresses issue #49.