#501 Tacticals are not cancelable

[Laurent Voisin]

In class BasicTactics, the tactics built by the tacticals never use the proof monitor parameter. This can get problematic when a tactical is used to create a long running tactic : this could lead to GUI freeze.
The tacticals shall react to cancellation. However, it is not clear what method "ITactic.apply()" shall do in case of cancellation. My first idea is that an exception shall be thrown so that the call can return fast. But this is not part of the current API. An important refactoring is needed here.