feat(validation): add manual validation to model

This is an experimental feature that enables validation for BPMN 2.0
diagrams via right click > Validate in work space and BPMN 2.0 editor.

It displays validation errors on BPMN 2.0 diagrams and in the Problems
tab.

Closes CAM-294
Merge branch 'master' of github.com:camunda/camunda-modeler