[Feature] Layer groups to organise layers

[alessandrofelder]

Is your feature request related to a problem? Please describe.

Implementing layer groups in napari has been discussed and attempted a few times in the past. It would be helpful for BrainGlobe-related plugins and likely others. A discussion in the community meeting in February 2024 suggested to implement this as a plugin in a separate repo first, because it is a relatively independent feature from the central code base but also quite experimental.

The idea is that once it stabilises, it would be easier to review and incorporate it into the main codebase.

• It might be useful to break this down into smaller issues.
• It's probably a good idea to rewrite something from scratch after reading the code in previous attempts.

Describe the solution you'd like

This issue will be addressed when there is implementation of the design discussed napari/napari#6345 (thanks, @isabela-pf) that people can interact with in a plugin.

Describe alternatives you've considered
Contributing this directly to napari, and including more complicated possible features of layer groups (controls like transforming all layers in the group at once) have been considered and discussed at community meetings, but have been deemed out of scope for now.

Additional context

Note that another experimental feature that could be implemented here first before making it into the napari codebase is the multi-canvas idea (@melonora)

[alessandrofelder]

Pushed my unfinished work (from February!) related to this to #2