Support storing calculated schedule

[Chris Nelson]

The TracPM component currently only does "soft" scheduling. It will add calc_start and calc_finish fields to the list of tickets it is passed but it doesn't provide anyway to save that schedule back to the database so, for example, a report can be sorted by those calculated dates.

The user can currently configure fields.start and fields.finish to specify the fields where the user-specified start or finish (due) dates are stored. In retrospect, this may be a bad choice of names. These configuration values might be better used for the calculated dates.

It seems I can:

• Rename the existing fields to user_* or due_* or something and user start and finish for the calculated values. Users would have to adjust their configuration.
• Keep start and finish the same and use scheduled_* or calculated_* for the output of the scheduler. I think this makes less sense.

I may also review PM literature to see if there's an industry standard way to distinguish these two types of dates.