Scale bar

[melonora]

References and relevant issues

depends on #6730
closes #6758

Description

This PR is another step in napari to go towards easily creating publication ready figures. The main problem with the current implementation of the scale bar is that it does not respond to canvas size. This means that if you were to do `viewer.screenshot(scale=8) the scale bar will show very small. This PR makes it so that the length of the scale bar responds to the horizontal canvas size and the width of the line segment and the font size responds to the vertical canvas size. Currently some of the settings have been made available in the experimental settings menu. This is not definitive, because @Olusesan and I would like a discussion first on the api / settings that should be made available to the user along with discussions of the scale bar interface in general.

We did remove the whiskers for now of the scale bar so the scale bar becomes more like the scale bar in Fiji. Main reason for that was because the height of the whiskers is another parameter, where the scale bar already has a few parameters.

This PR was also so far done in paired coding sessions with @Olusesan.

[codecov]

Codecov Report

Attention: Patch coverage is 90.69767% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 92.42%. Comparing base (35f5b12) to head (3fd75a5).
Report is 98 commits behind head on main.

Files	Patch %	Lines
napari/_qt/qt_main_window.py	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6892      +/-   ##
==========================================
+ Coverage   92.35%   92.42%   +0.07%     
==========================================
  Files         605      614       +9     
  Lines       54491    55195     +704     
==========================================
+ Hits        50325    51014     +689     
- Misses       4166     4181      +15     

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