refactored constraint checker service to use service base implementation

org.fortiss.tooling.kernel/trunk/plugin.xml

...ng.kernel/trunk/schema/modelElementConstraintChecker.exsd

...fortiss/tooling/kernel/interfaces/IConstraintChecker.java

...rtiss/tooling/kernel/interfaces/IConstraintViolation.java

...rg/fortiss/tooling/kernel/internal/ConstraintService.java

...g/fortiss/tooling/kernel/services/IConstraintService.java