Add pyside-lupdate support for translatorcomment tags.

This includes changes to TS parsing and saving, as well as the
[Meta]TranslatorMessage data classes.  The translator comment must be stored
in the data model so that it is propagated through a "merge", but it is not
part of the key used to identify a translation, so it is not involved in
equality tests or comparisons.
Add pyside-lupdate support for lengthvariant tags.

This is a simple change to TS parsing and saving; no changes are required in
the data model.  Translation variants are simply concatenated with a special
separating character, and Qt knows how to handle the display of the
concatenated string.