feat: add QFlowLayout, for variable width widgets

[tlambert03]

This adds the QFlowLayout described at https://doc.qt.io/qt-6/qtwidgets-layouts-flowlayout-example.html

Untitled.mov

[codecov]

Codecov Report

Attention: Patch coverage is 91.95402% with 7 lines in your changes missing coverage. Please review.

Project coverage is 87.27%. Comparing base (6a7a731) to head (edbc5c9).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/superqt/utils/_flow_layout.py	91.76%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
+ Coverage   87.15%   87.27%   +0.11%     
==========================================
  Files          46       47       +1     
  Lines        3465     3551      +86     
==========================================
+ Hits         3020     3099      +79     
- Misses        445      452       +7     

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