Allow editing of date & time and air & water temperatures

Add two more rows to the widget - this is getting quite busy.
There still is some weirdness where the focus isn't returned where it
should be and a few other details, but overall getting there.

Added helper functions to parse a temperature and to deal with the
timezone offset - with that latter one I also fixed the time offset bug in
the planner.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>