Provide checkers for ecore/ALE/k3/... entries (via extension point) for dsl file content

The new dsl file does not report any error or warning for files such as

```
name = org.eclipse.gemoc.example.k3fsm.K3fsm
ecore = platform:/resource/ecoreFilePath,platform:/resource/org.eclipse.gemoc.example.k3fsm/model/k3fsm.ecore
k3 = qualified.class.name
```

As dsl file is an extensible format we need to set up a mechanism so the check can be contributed by component.

In this case: the general GEMOC framework should provide a check about the ecore entry and the k3 entry in order to verify that it correctly indicates a existing resources.