Reuse value format logic for label width determination in sliders

[brisvag]

• closes QLabeledDoubleRangeSlider label width too small #335

There is still a small overlap between the handle label text if they are both long. Not sure why that's the case. At least it solves the "cropping" problem from the original issue.

Before:

After:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.96%. Comparing base (595b284) to head (dd91e78).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
+ Coverage   84.85%   84.96%   +0.11%     
==========================================
  Files          49       49              
  Lines        3909     3912       +3     
==========================================
+ Hits         3317     3324       +7     
+ Misses        592      588       -4     

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[tlambert03]

thanks @brisvag