difference-equation: add a constructor field

The modeling plugin has now a new field that enable to add custom code
into the constructor method. From now on, this way, we can define
custom parameters.
(closes #50)
vle.forrester: fix the timeStep Button

Also fix the Include button. Connection signals were destroyed much to
soon.
vle.forrester: fix the time step management

For the compartment dialog and the pure time step dialog, control and
visualisation of doubles have been improved.