dtd: Use source accesskey if target accesskey is missing
dtd: don't remove stray & as they probably &amp;

Since we now convert & -> &amp; we no longer need to worry about these busted
ampersands.
accesskeys: don't use space as an accesskey
Properly detect all Gaia plural units

Not all Gaia plural units start with “plural(n)”, in particular “n” is
the variable designated by the developer to control which plural rule to
be selected, so it can be anything. Now our code checks for Gaia plurals
in a more generic way.