Modernize custom open project dialog

[ptsavol]

• Make browsing folders much quicker when mounted or network drives are present
• Show busy cursor when expanding folders

No associated issue.

Checklist before merging

• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

❌ Patch coverage is 70.58824% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.59%. Comparing base (aaa4190) to head (9ffbeab).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
spinetoolbox/widgets/open_project_dialog.py	70.58%	11 Missing and 9 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3322      +/-   ##
==========================================
- Coverage   70.60%   70.59%   -0.01%     
==========================================
  Files         208      208              
  Lines       40292    40320      +28     
  Branches     4880     4888       +8     
==========================================
+ Hits        28447    28463      +16     
- Misses      10505    10511       +6     
- Partials     1340     1346       +6     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.