Fix issue where collapsed categories were not remembered after toggling a track

[cmdcolin]

The title of this PR somewhat underestimates the issue scope but it is what the end user experiences

The collapsed category state was not stored in the widget model properly, and it was causing the code to forget the collapsed categories after a track is toggled (which causes the tree to get rerendered)

Note: Technically it might be somewhat expensive/not the best way to do things for for the tree to get recalculated after things like a track toggle or category collapse, but it seems to work fine afaik

This change is also good because it is nice for the collapsed categories to also be remembered in state e.g. across page refreshes

Fixes #2207

[codecov]

Codecov Report

Merging #2208 (2fbf4c6) into main (bd3b7b1) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2208      +/-   ##
==========================================
+ Coverage   62.50%   62.56%   +0.05%     
==========================================
  Files         484      484              
  Lines       22707    22709       +2     
  Branches     5152     5154       +2     
==========================================
+ Hits        14194    14208      +14     
+ Misses       8243     8231      -12     
  Partials      270      270              

Impacted Files	Coverage Δ
...ement/src/HierarchicalTrackSelectorWidget/model.js	61.41% <ø> (+1.57%)	⬆️
...ctorWidget/components/HierarchicalTrackSelector.js	61.39% <100.00%> (+0.49%)	⬆️
packages/core/util/index.ts	79.84% <0.00%> (ø)
...nts/src/SNPCoverageRenderer/SNPCoverageRenderer.ts	87.65% <0.00%> (+7.40%)	⬆️
packages/core/util/when.ts	54.05% <0.00%> (+10.81%)	⬆️

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