This change does two things:
1) Changes the "charset" property to be an IPuzCharset, and adds
"charset-str" to be the string prepresentation of the charset
2) Creates one at Puzzle creation time, and keeps those two strings in
sync whenever they change. If one changes, the other is synced to the
new one.

This will guarantee that every IPuzCharset will have a charset when
its created, and will save it when done