Add action to trigger validation

[eneufeld]

modelserver-theia offers the possibility to populate the problem view from the model server.
See eclipse-emfcloud/emfcloud-modelserver-theia@06c9b49#diff-62f187394797d135f071e49d4a4d36d1c133b44cf4b302a5ae41ccb5efc99ddf

[eneufeld]

Needs to be tested what happens if glsp is opened after the validation action was run.