UML to CIF translation: add stereotype for occurrence constraints

[AndreaPuffo]

Very often we add occurrence constraints to limit the number of times an activity can call a behavior (either opaque behavior or another activity).
Adding a new occurrence constraint is quite annoying: a user needs to create a new Interval Constraint, click on the UML elements to constrain, add an Interval child specification, create Literal Integers representing the max/min number of calls, and link them to the child specification.
We should create a dedicated stereotype, a new SynthML tab that governs all these aspects easily (or, at least, in a less complex way).

[AndreaPuffo]

The PR #675 adds a stereotype for occurrence constraints that handles max/min interval bounds.
A follow-up (see discussion https://github.com/TNO/SynthML/pull/675/changes/BASE..c67a03ab16a747786d1e9648e6e7ab8c2e57ac4e#r2665082386) would be to add the constrained element in the SynthML tab. This means adding a "selection" widget (not sure if "selection" is the correct name) to select the to-be-constrained behaviors.