Visualize speaker self-overlaps in the details plot

[thequilo]

Fixes the (long broken) warning for self-overlap in the UI and splits a speaker column into multiple sub-columns if utterances of the same speaker overlap.

This is an example of the overlap visualization in the details plot

This way of visualization is not pretty since the lines connecting matching words overlap, but it shows all relevant information and is much better than the previous behavior.

I also cleaned up the code a little:

• Removed the partial support for multiple hypothesis in the same visualization in favor of the synchronized side-by-side view
• The horizontal positions of the utterances are now precomputed
• The WER displayed in the visualization now matches the WER computed by the CLI tool exactly
• It is now easier to add assignment algorithms, like DI-cp, in the future
• Added buttons to quickly jump to the next/prev utterance with self-overlap