[DMN Editor] Decision service

The DMN Editor is to support Decision Services (as defined in the DMN 1.2 Specification).

Decision Services should present themselves as a new node in the Palette that can be dragged on to the Canvas.

Decision Services will appear expanded to begin however we should also add support for collapsing the Decision Service node (thus hiding it's content).

A Decision Service node acts as a container for the other pre-existing DMN elements (Decision, BKM, InputData, KnowledgeSource etc).

See 5.3.3 Decision services of the DMN 1.2 Specification for details.

See 6.2.3 Connection rules for connection rules.

See 6.2.5 Decision service for schematics.