#1319 - Cross-Layer Relations

[reckart]

What's in the PR

• Handle relation creation fully in the CreateRelationAnnotationHandle and to not go through the AnnotationDetailEditorPanel
• Open context menu if there are multiple relation annotations that could be created between two span layers
• Make context menu accessible by other handlers than the ShowContextMenuHandler
• Move getRelationLayersFor to the AnnotationSchemaService

How to test manually

• Enable experimental cross-layer creation
• Create two relation layers that attach to Any span layer
• Try using them

Automatic testing

• [ ] PR includes unit tests

Documentation

• [ ] PR updates documentation