feat: Use the Slivers mechanism with the Gallery view

[g123k]

Hi everyone,

As explained in #4912, the Box mechanism in Flutter ensures that ALL Widgets are built, even if they are not visible by the user. For that, we have Slivers and their lazy loading capacity.

This PR moves the code to Slivers and also simplifies it by not relying on rows of 2 items.

A demo:

Untitled.mov

[codecov-commenter]

Codecov Report

Attention: 119 lines in your changes are missing coverage. Please review.

Comparison is base (fe7e1f8) 9.67% compared to head (273b2d8) 9.66%.

Files	Patch %	Lines
.../lib/pages/product/product_image_gallery_view.dart	0.00%	70 Missing ⚠️
packages/smooth_app/lib/widgets/slivers.dart	0.00%	25 Missing ⚠️
.../pages/image/product_image_gallery_other_view.dart	0.00%	21 Missing ⚠️
...p/lib/generic_lib/widgets/images/smooth_image.dart	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #4913      +/-   ##
==========================================
- Coverage     9.67%   9.66%   -0.01%     
==========================================
  Files          318     319       +1     
  Lines        16115   16125      +10     
==========================================
  Hits          1559    1559              
- Misses       14556   14566      +10     

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[teolemon]

thanks @g123k and @monsieurtanuki 👍