Fix disappearing "ID mapping" dropdown

[hotzenklotz]

PR fixes a bug where the ÌD Mapping` dropdown of a segmentation layer was hidden when the layer opacity is set to 0.

Possible cause of the bug:

• Previous versions of the wK UI had the "ID Mapping" dropdown/UI separate of the Segmentation Layer settings. I.e. the ID mapping dropdown had to "disable"/hide itself, when the Segmentation layer was disabled.
• Since moving the ID mapping setting into the Segmentation layer setting, this behavior is no longer required.

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• Set Segmentation layer opacity to 0
• Toggle the segmentation layer to refresh React rendering --> ID mapping dropdown should still be visible
• Toggle the ID mapping toggle to refresh React rendering --> ID mapping dropdown should still be visible

Issues:

• fixes Mapping UI disappears when changing opacity of layer to 0 #6210

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

• Updated (unreleased) changelog
• Updated (unreleased) migration guide if applicable
• Updated documentation if applicable
• Adapted wk-connect if datastore API changes
• Adapted wk-libs python client if relevant API parts change
• Needs datastore update after deployment
• Ready for review

[philippotto]

Nice 👍