itex2MML 1.3.12: \coloneqq and its cousins

Support for:

\coloneqq
\Coloneqq
\coloneq
\Coloneq
\eqqcolon
\Eqqcolon
\eqcolon
\Eqcolon
\colonapprox
\Colonapprox
\colonsim
\Colonsim
\dblcolon

Not all of these have corresponding Unicode
characters, so may appear a little hackish.
Also, Firefox doesn't seem to set the correct
spacing for binary operators for most of these.
So we hhave to set that by hand.