Prevent the creation of invalid hops in a visual manner #6281

[nadment]

• Add a visual indicator if a hop candidate already exists
• Add a visual indicator when a hop candidate create a loop in workflow or pipeline
• Avoids side effects when drawing a new hop candidate and clicking on a action/transfo name, hop, or note.
• Reduce the size of the red cross.
• Some doc cleanup
• The visual indicator does not take into account the offset in the pipeline graph

@hansva can you test if there is no duplicate pop-up for hop handling, I removed some Mac-specific code at line 738 of PipelineGraph.

[hansva]

Looking good, I have not found any issues