Implement widening across multiple executions of collapse

[marcelosousa]

Within a configuration, we would like to apply widening even if multiple executions of collapse have been issued. That is the only way to achieve termination of the analysis.
There are two possibilities to implement this feature:

1. In the collapse by passing extra information, e.g. the CFG annotated with the previous states.
2. In the unfolding algorithm, by keeping track per configuration a Map from EventName to IntState.

[marcelosousa]

Widening needs to return actions as well.