Threshtime: Rewrite the converter dialog almost entirely.

Instead of having three radio buttons "Local", "EST", "EDT", it now has two
separate RL-time fields, for local time and America/New_York time; thus it
uses and depends on Olsen tzdata, rather than on the human picking the right
one. Also, instead of having explicit "Convert" buttons (which looked ugly),
it simply updates on every edit. The previous version was a slightly buggy
port of the RosMud time converter; this version stands separately as a usable
time converter.