fix: prevent flicker on dims_changed

[tlambert03]

this fixes unnecessary canvas clearing on dims_changed, and prevents the width of the qt slider labels from jiggling as they change.

before:

Untitled2.mov

after:

Untitled.mov

[codecov]

Codecov Report

❌ Patch coverage is 70.83333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.01%. Comparing base (5da7236) to head (c7ed8ef).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ndv/controllers/_array_viewer.py	30.00%	7 Missing ⚠️

❌ Your patch status has failed because the patch coverage (70.83%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #231      +/-   ##
==========================================
- Coverage   86.29%   86.01%   -0.28%     
==========================================
  Files          46       46              
  Lines        5209     5228      +19     
==========================================
+ Hits         4495     4497       +2     
- Misses        714      731      +17     

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