Add support for temporal relationships between tasks (reltype=depends-on)

[eluap-lab]

Hi!
Thanks for the great app! I really enjoy using it.

One great feature to add would be temporal relationships, e.g. finish to start, i.e. this task begins only after the previous task is finished). Specified here: https://www.ietf.org/archive/id/draft-ietf-calext-ical-relations-05.pdf#file%3A///var/tmp/CGItemp5096.dir/draft-ieft-calext-ical-relations.html-2.html%23redefined_relation_type_value

This would also allow to add support for gantt charts or a gantt view in the app.

Thanks!