Manage atlas versions in a separate widget

[alessandrofelder]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
We'd like to separate atlas version management from atlas visualisation in the GUI.

What does this PR do?

• creates a new widget for atlas version management
• in the background, separates models/views into their own files (as we now have more than one view for the same model)

References

Closes #21

How has this PR been tested?

Please explain how any new code has been tested, and how you have ensured that no existing functionality has changed.

Is this a breaking change?

If this PR breaks any existing functionality, please explain how and why.

Does this PR require an update to the documentation?

If any features have changed, or have been added. Please explain how the documentation has been updated (and link to the associated PR). See here for details.

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[codecov]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (94aff08) 97.44% compared to head (b0609a0) 96.78%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
- Coverage   97.44%   96.78%   -0.66%     
==========================================
  Files           8       12       +4     
  Lines         391      467      +76     
==========================================
+ Hits          381      452      +71     
- Misses         10       15       +5     

Files	Coverage Δ
brainrender_napari/__init__.py	100.00% <100.00%> (ø)
brainrender_napari/atlas_version_manager_widget.py	100.00% <100.00%> (ø)
brainrender_napari/brainrender_widget.py	100.00% <100.00%> (ø)
...rainrender_napari/data_models/atlas_table_model.py	100.00% <100.00%> (ø)
brainrender_napari/utils/brainglobe_logo.py	100.00% <100.00%> (ø)
brainrender_napari/widgets/atlas_manager_dialog.py	100.00% <100.00%> (ø)
brainrender_napari/widgets/structure_view.py	100.00% <100.00%> (+6.61%)	⬆️
brainrender_napari/widgets/atlas_viewer_view.py	98.14% <98.14%> (ø)
brainrender_napari/widgets/atlas_manager_view.py	88.23% <88.23%> (ø)
...nrender_napari/data_models/structure_tree_model.py	90.69% <90.69%> (ø)

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[alessandrofelder]

Closing in favour of a few smaller PRs.