Task list navigation and display style.
Reuse TaskView when possible.
Rename task list column.
Use weakrefs for model to view mapping.

Once the model object is gone, we do not need to keep the view any
longer.
Keep focus in task list on right element on update
Show newest finished tasks on top.
Allow to terminate tasks.
Improved ModalWidget.

Does not require the parent to be passed in, but the mainloop has to be
set first.
Make width and height of ModalWidget controlable.
Ask confirmation before killing tasks.