OrderedTask: update TaskAdvance in SetFactory()

Previously, loading a task left the TaskAdvance object in whatever
state was configured for the default task.
InputEventsTask: translate task start message
TaskStats: remove unused attribute "Time"

It is not clear why this attribute is useful.
TaskStats: make "flight_mode_height_margin" a constant

This was never accessed except inside TaskStats.
TaskStats: simplify margin formula

No integer multiplication.
TaskStats: move task_started to new struct StartStats
StartStats: add attributes time, altitude, ground_speed

Replaces ProtectedTaskManager::GetStartState().  The new method is
lockless.
OrderedTask: remove unused method GetFinishState()
AbstractTask: eliminate GetFinishHeight()

The only caller was RulesStatusPanel::Refresh(), and this was
incorrect, because this function must be hard-coded to use the ordered
task, not the active task.
RulesStatusPanel: allow the TaskManager to be nullptr

Fill the attributes we do know from TaskStats.
StartStats: add method SetStarted()
GotoTask, AbortTask: move CheckTransitions() to UnorderedTask

Merges two identical implementations into the base class.
UnorderedTask: set StartStats.task_started
AbstractTask: use TaskStats instead of TaskStarted/Finished()
OrderedTask: use SetOrderedTaskBehaviour() in Commit()

Fixes the start open/close update.
StartPoint: enforce the start open/close time

Implements one part of TRAC #2678.  The second part of that ticket
(option "on entry" or "on exit") is described by TRAC #2099.
release v6.6_beta1
test/RunWindComputer: new debug program
AUTHORS: add Frank Paynter and Frank van der Hulst
test/RunCirclingWind: reset the computer objects

Make valgrind happy.