[DMN Designer] Decision Service splitter should be movable

The splitter within a Decision Service is static.

It needs to be movable (vertically) maintaining it's alignment with the Decision Service bounds. At this point we need to either decide to add result decision(s) and encapsulated decisions to the Decision Service by Command (depending on where the node is dropped) or update the Marshaller to determine the type of decision based on its vertical positioning and location of the splitter.

Acceptance test

Decision state is updated according to splitter movement. Example if area of result decision expanded so some decision belongs there now, this decision should have state rusult decision