Task views now include the list of dependencies and reverse
dependencies. In addition, it is impossible to mark a task as completed
if it has unsatisfied dependencies, and it is impossible to re-activate
a task that has completed reverse dependencies.
This initial import is a heavily modified version of the code I had
here, as Arse was modified for other purposes in the meantime and the
application no longer worked with it.
In addition:
* I did not import the user management part yet,
* task dependencies are supported in-base, but there is no interface
for that yet.
