The expression editor options should be dependent on the diagram node type.

Decision Node

Should allow all expression types except of Function

BKM Node

Should allow only Function expression type.

Further improvement

BKM node has automatically function expression inside - separate jira, or won't be implemented?
(jomarko@redhat.com IMO If/when etirelli confirms the subject of this JIRA is the expected/required operation I'd ensure BKMs have a Function set and cannot be cleared).

Acceptance tests

• Not possible to change top level function expression type for BKM node
• Possible to change not top level function expression type for BKM node
• Possible to change any level function expression type for Decision node