Add reset settings buttons

[ptsavol]

Adds a button into File->Settings for restoring settings to factory defaults.

Fixes #3018

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 & isort
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 74.07407% with 21 lines in your changes missing coverage. Please review.

Project coverage is 67.15%. Comparing base (206f459) to head (cdcff96).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
spinetoolbox/widgets/settings_widget.py	30.00%	14 Missing ⚠️
spinetoolbox/ui_main.py	33.33%	3 Missing and 1 partial ⚠️
spinetoolbox/helpers.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3019   +/-   ##
=======================================
  Coverage   67.15%   67.15%           
=======================================
  Files         193      193           
  Lines       37024    37061   +37     
  Branches     4443     4445    +2     
=======================================
+ Hits        24862    24887   +25     
- Misses      10908    10918   +10     
- Partials     1254     1256    +2     

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