fix(validation): activate validation on manual validate only

This commit disables automatic project validation for BPMN 2.0 diagrams.

Diagrams may still be validated via right-click -> validate on
project, bpmn diagram or bpmn diagram file.

Closes CAM-1508
feat(validation): rework BPMN 2.0 validation rules

This commit removes a number of wrong and meaningless validation rules
and changes the status of the remaining validations to WARNING level
(was ERROR for most).

Related to CAM-1508