[ENH] widgetsscheme: Schedule gc.collect on widget delete

[ales-erjavec]

Issue

Fixes: #288

Description of changes

Schedule gc.collect on widget delete

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.07%. Comparing base (600f194) to head (41e3f17).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
+ Coverage   72.04%   72.07%   +0.02%     
==========================================
  Files          34       34              
  Lines        9115     9124       +9     
==========================================
+ Hits         6567     6576       +9     
  Misses       2548     2548              

☔ View full report in Codecov by Sentry.
📢 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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.