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 #3002.
• Added a troubleshooting page for the error message in #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

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

[codecov[bot]]

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 :warning:
spinetoolbox/widgets/settings_widget.py	0.00%	2 Missing :warning:
spinetoolbox/helpers.py	66.66%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ 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 ```

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