[gaia] Properly detect all 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.
[gaia] Keep special cases separately translatable

Gaia will use [zero], [one] and [two] cases for all locales, regardless
of the locale's plural rule, so we need to be able to translate them
separately.