Proposal: refresh all sirius layers by default + simplify declaration of layers to refresh

At the moment, the GEMOC animator requires the language engineer to explicitly declare which layers to refresh by extending the `AbstractGemocAnimatorServices` class. Example:
```java
public class FsmAnimatorServices extends AbstractGemocAnimatorServices {
@Override
protected List getRepresentationRefreshList() {
final List res = new ArrayList();
res.add(new StringCouple("FSM", "Animation"));
return res;
}
}
````

This leads to a few problems:
- If he/she forgets to declare such layers, there is no animation at all by default
- It's a little tedious to write java code just to declare a couple of string
- There is no checking/feedback on whether the declared layers do exist and are valid

Proposal:
- Refresh all animations layers by default (change to make in `AbstractGemocAnimatorServices`)
- Allow the language engineer to override this default behavior by declaring which layers should only be refreshed, and allow him/her to do that in the `.dsl` file instead of using Java code. This could in addition give the possibility to add an Xtext checker to verify whether the given layers are valid.