Improve layout capabilities for flow source and sink specifications.

[philip-alldredge]

A long standing limitation of the graphical editor has been the inability to move the start anchor for flow sources and sinks. Because of this, flow sources and sinks are not considered during the automatic layout process. The support for EMV2 in #1546 will need improved support for similar diagram elements.

• Add support for having different ending points for each flow source/sink.
• Add support for moving ending anchor.
• Add support for persisting the location of the ending anchor.
• Add support for automatic layout of such anchors and flows.
• Update User Guide and Known Layout Issues and Limitations #1192 to remove the reference to flow source and sink layout limitation.

[philip-alldredge]

It is not practical to allow the user to move the anchors with the current graphiti implementation of the graphical editor. A new issue can be opened in the future if that is requested. The other parts of the issue will be attempted with the current editor.

[philip-alldredge]

This issue has been implemented. Due to Graphiti constraints and general graphical editor architecture, it isn't as flexible as preferred but it is a major step in the right direction. Other issues such as not being able to manually adjust flow indicator position will be resolved as part of a more significant update. Will open a pull request once a preferred nightly ELK build has been selected.