Combine custom names of merged agglomerates & transfer custom name to split-off segments

[MichaelBuessemeyer]

This PR improves the way custom names of segments / agglomerates are handled during proofreading. Previously, the potential custom name of the target segment was dropped in a merge action and a split-off segment always "lost" its previous custom name. Now, the custom names are merged in case of a merge and split-off segments inherit the custom name of the original agglomerate.

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• Open a dataset with a segmentation layer with a hdf5 mapping.
• load 2 segments & give each a custom name
• merge these segments. The newly merged segment should have both custom names separated by a comma.
• Merge two segments out of which only one has a custom name. The merged segment should have retained the custom name but not the default one.
• Split of a segment from an agglomerate with a custom name. The split-off segment should have the same name as the original agglomerate.
• Split a segment from all its neighbors. All new split off neighbors should have the custom name of the original segment in case the original segment had a custom name.

TODOs:

• Check whether there are other proofreading actions that should be considered.

Issues:

• fixes #7830

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(Please delete unneeded items, merge only when none are left open)

• Updated changelog

[philippotto]

code lgtm 👍 didn't test because I assume that you tested the mentioned proofreading operations well.

[MichaelBuessemeyer]

just did another quick test on the dev instance to avoid a big accidental woopsie. Testing worked 👍