feat: calculate start date based on dependencies

[lettuceDestroyer]

If the start date of a task is not set it is calculated based on the dependencies. Meaning the start date will be the end date of the dependency. If a task has multiple dependencies the "bigger" end date will be taken as start date.

Please not that this feature does not work correctly if you do not have the duration of a task set. To enable duration please refer to either this or this pull request.