More flexible timing UI [1715199]

[drachefly]

Instead of start and finish fields, how about three fields -- start,
duration, finish. These are not forced to be consistent as you fill them
in.

Each has a 'fill' button to its right, which fills in the proper value for
that field given the values in the other two fields.

You can only save if all fields are consistent or if exactly one field is
empty.