Make ValidationTool a toggleable tool

[tanjaem]

Per default, the ValidationTool should not be active and there should be no validation markers (info, warning or error) shown in the diagram.
Once the user activates the ValidationTool, the model should be validated with every model change and the resulting validation markers should be shown in the diagram.
When the user de-activates the ValidationTool, all validation markers should be removed from the diagram and no more validations should be executed.

Related to issue #30

[tortmayr]

Sounds good. Maybe the validation tool should also be selection-aware to compute the modelIds for the RequestMarkersAction