The start field is a computed field but it doesn't have any inverse function
(write() is overridden in calendar.event to, among other things, play the
role of the missing inverse function). It is thus automatically set as
readonly, and the events can't be dragged and dropped in the calendar view,
since rev. d6d3d098 in which we start checking if the start_date is readonly
or not before allowing the drag and drop.
This rev. simply adds a "do_nothing" inverse function so that the date field
is not readonly.
In web_calendar, the date_start attribute is mandatory so fallbacking on
date_end and date_delay is unnecessary.
Aaron Bohy
40971b1b4f