[Planner Dev] cost of tasks with subtasks

[Doroszlai Attila <eti pga hu>]

Hi,

I'm working on the caching of task cost (part of #138475) and I
need some advice.

1. The cached value has to be invalidated when assignments get
   added or removed, and when task duration (or work?) is changed.
   I've solved this with a function in mrp-task that marks the
   cached value as stale, so that on the next request, cost is
   recalculated.  Staleness also propagates to parent tasks.  This
   function is called from

   - task_assignment_removed_cb,
   - task_remove_assignments,
   - imrp_task_add_assignment,
   - imrp_task_set_duration.

   Question: is this OK, or is there another, preferred way to do
   it?

2. Invalidation also must happen when the cost of a resource is
   changed.  For this, I added a function that invalidates the
   cached cost for all tasks that a resource is assigned to.  This
   is called from resource_dialog_cost_changed_cb in
   planner-resource-dialog.c.

   Question: how do I handle the cost-change event if it happens
   in the resource view, not the dialog?  The problem is that in
   the view, cost is a custom property, while for the dialog,
   there is a callback explicitly for cost change.

-Attila