Improvement: Use on_progress and on_date_change as validity checks

[KebabRonin]

Currently, the on_progress and on_date_change events are triggered after a task has been updated. As such, the secod/third parameters, presumably containing the new values, are redundant (they are the same as their respective fields in the task object).

A fix for this would be to trigger the event before the update, or to create a new event that gets triggered before as opposed to after a change.

Additionally, the callbacks could return a true or false depending on if the change should continue. On a false, the task is reverted back to its state before starting the change.

Example:

If I want to implement a validation check for the new date (like not allowing a task to start on certain days, or not allowing any progress changes), I can't use the old values as part of that check or to revert the bar to it's previous state on a failure.