Allow to make a layer exclusively visible by pressing ctrl when changing visibility

[philippotto]

URL of deployed dev instance (used for testing):

• https://ctrlvisibilitytoggle.webknossos.xyz

Steps to test:

• open a dataset with multiple colors layers
• click the visibility switch while holding ctrl
• the layer should be the only visibile layer
• selecting another layer with ctrl should make that layer the only visible one
• using ctrl on the currently exclusively visible layer should make all layers visible again

the toggling behavior is the same as in GIMP (and I think other major image editors)

Issues:

• fixes Make it easy to exclusively render one data layer #3896

---

• Updated changelog
• Updated documentation if applicable
• Ready for review

[daniel-wer]

Works very nicely, code looks good too :)

If you find a good spot, it would be cool to document this behavior in the docs (as a pro-tip, or did-you-know or so)!