Update Add/Update SpineOpt Wizard

[ptsavol]

This PR fixes some issues with the SpineOpt installer wizard that opens from the Add/Update SpineOpt button in Spine Toolbox's File->Settings->Tools menu.

• The wizard does not add SpineJuliaRegistry anymore. SpineOpt is installed from the General registry. This should fix issue SpineOpt Installation Wizard fails when Toolbox runs in a Conda environment on Windows #3002.
• Added a troubleshooting page for the error message in SpineOpt Installation Wizard fails when Toolbox runs in a Conda environment on Windows #3002 if it should appear again.
• The warning message Warning: The Pkg REPL mode is intended for interactive use only... does not appear anymore.
• The julia project lineedit in File->Settings->Tools must be an existing directory, @. or empty now. Previously, it was possible to just write anything there and the wizard would create the environment in spine toolbox's root dir.
• Bumped the REQUIRED_SPINEOPT_VERSION to 0.9.0
• Fixed some buttons on troubleshooting pages
• Cancel buttons are not shown on wizard's final pages anymore.

Fixes #3002

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 89.83051% with 6 lines in your changes missing coverage. Please review.

Project coverage is 67.15%. Comparing base (3599115) to head (d47ff3f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
spinetoolbox/widgets/add_up_spine_opt_wizard.py	93.87%	1 Missing and 2 partials ⚠️
spinetoolbox/widgets/settings_widget.py	0.00%	2 Missing ⚠️
spinetoolbox/helpers.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3005      +/-   ##
==========================================
+ Coverage   67.13%   67.15%   +0.02%     
==========================================
  Files         193      193              
  Lines       37102    37138      +36     
  Branches     4445     4450       +5     
==========================================
+ Hits        24909    24941      +32     
- Misses      10937    10939       +2     
- Partials     1256     1258       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.