fix: 3436 - no more "serving size" jump

[monsieurtanuki]

What

• When we changed a nutrient unit, the whole page was refreshed. And there was a side-effect with the focus nodes that got recomputed.
• The fix was to refactor in several Widgets. More specifically, with a StatefulWidget for the nutrient unit, that only refreshes itself.

Fixes bug(s)

• Fixes: Nutrition Facts Page: view jumps to empty Serving size field when clicking unit buttons #3436

Impacted file

• nutrition_page_loaded.dart: refactored with "real" Widgets

[codecov-commenter]

Codecov Report

Merging #3921 (a0b4d11) into develop (e5bf1cb) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #3921      +/-   ##
===========================================
- Coverage    10.64%   10.63%   -0.01%     
===========================================
  Files          271      271              
  Lines        13478    13489      +11     
===========================================
  Hits          1435     1435              
- Misses       12043    12054      +11     

Impacted Files	Coverage Δ
...h_app/lib/pages/product/nutrition_page_loaded.dart	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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[M123-dev]

Looks good, thanks @monsieurtanuki

Related: #2621

[monsieurtanuki]

Thank you @M123-dev for your review!

Indeed that's why splitting in different Widgets and possibly different files makes sense.