Fix fully collapsable widgets

[PiispaH]

Disabled fully collapsing widgets by setting every QSplitters childrenCollapsible property to false.

Fixes #2167

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black
• Unit tests pass

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03% ⚠️

Comparison is base (af55312) 63.81% compared to head (a5b9667) 63.79%.
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2244      +/-   ##
==========================================
- Coverage   63.81%   63.79%   -0.03%     
==========================================
  Files         198      198              
  Lines       35942    35947       +5     
  Branches     6114     5121     -993     
==========================================
- Hits        22937    22932       -5     
- Misses      11779    11790      +11     
+ Partials     1226     1225       -1     

Files Changed	Coverage Δ
spinetoolbox/ui/array_editor.py	98.78% <100.00%> (+0.01%)	⬆️
spinetoolbox/ui/mini_kernel_editor_dialog.py	99.16% <100.00%> (+<0.01%)	⬆️
spinetoolbox/ui/settings.py	99.87% <100.00%> (+<0.01%)	⬆️
...etoolbox/ui/time_series_fixed_resolution_editor.py	98.82% <100.00%> (+0.01%)	⬆️
...olbox/ui/time_series_variable_resolution_editor.py	97.87% <100.00%> (+0.04%)	⬆️

... and 2 files with indirect coverage changes

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