Exit a list in adding/editing tasks cancels the edit mode.

Before TaskTableController changes content, cancel all rows we are
editing. This will in turn re-enable the "Add Task" if it was in a
disabled state (if user where in the middle of adding a task).

Maybe we should ask the user if he/she wants to actually leave if they
are editing tasks?
Fixed weakness in disable/enable Add Taks-button.

Problem was that we did not remove the observer,
so if the user added a new task 'A', save that task
and add another one clicking on Task 'A' would
still interfere on the state for Add Task button.
Added new idea to a thing todo ;)
Merge branch 'master' into pg-sql