Backend multichannel

[melonora]

References and relevant issues

Will add all these later

Description

This PR implements the initial backend changes required for having multichannel view (multiple viewboxes with synchronized cameras). There are some errors that are different when I worked / implemented this previously, mainly with respect to the cameras. I am currently working through those issues.

[codecov]

Codecov Report

Attention: Patch coverage is 25.86207% with 43 lines in your changes are missing coverage. Please review.

Project coverage is 92.36%. Comparing base (a9c25e6) to head (0162c4a).
Report is 16 commits behind head on main.

Files	Patch %	Lines
napari/components/grid.py	19.35%	25 Missing ⚠️
napari/_vispy/canvas.py	21.73%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6902      +/-   ##
==========================================
- Coverage   92.45%   92.36%   -0.10%     
==========================================
  Files         614      612       -2     
  Lines       55166    55221      +55     
==========================================
+ Hits        51002    51003       +1     
- Misses       4164     4218      +54     

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