add tooltip hints to UI

[alessandrofelder]

Description

What is this PR

• [ ] Bug fix
• [x] Addition of a new feature
• [ ] Other

Why is this PR needed? It should be as easy as possible for users to use brainrender-napari

What does this PR do?

• Adds tooltips for all widgets, containing simple user instructions for each of them
• Divides the atlas and the structure widgets into two groups, so it's clearer which belongs to which.

References

Closes #52

How has this PR been tested?

• new tests for tooltips
• tests pass locally and on CI
• visual inspection by hovering over widgets

Is this a breaking change?

No

Does this PR require an update to the documentation?

No

Checklist:

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

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.09% :tada:

Comparison is base (fb9d764) 96.95% compared to head (cee970e) 97.04%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #79 +/- ## ========================================== + Coverage 96.95% 97.04% +0.09% ========================================== Files 7 7 Lines 328 338 +10 ========================================== + Hits 318 328 +10 Misses 10 10 ``` | [Files Changed](https://app.codecov.io/gh/brainglobe/brainrender-napari/pull/79?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=brainglobe) | Coverage Δ | | |---|---|---| | [brainrender\_napari/brainrender\_widget.py](https://app.codecov.io/gh/brainglobe/brainrender-napari/pull/79?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=brainglobe#diff-YnJhaW5yZW5kZXJfbmFwYXJpL2JyYWlucmVuZGVyX3dpZGdldC5weQ==) | `100.00% <100.00%> (ø)` | |

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