Filter workflow nodes that are not user tasks in designer

Currently in designer the following workflow nodes are being filtered out due to not having layouts: Start, Stop, Fork 2 ways, Fork 3 ways and Merge.

This filtering is simple because workflow nodes have properties such as startNode, endNode, forkNode and mergeNode.

However, the Node and Sub Workflow nodes also don't have layouts and are still shown (currently, workflow nodes don't seem have properties to filter these trivially).

Note: The validation errors in the screenshots are simply due to the nodes not being linked and the Sub Workflow node not having a workflow set.