Robustify and simplify View.has_finished() logic

[mattpap]

The goals of this PR are:

• use View.children() in View.has_finished() instead of overriding has_finished() in subclasses
• make sure that all views participate in has_finished() and are marked as finished at some point in their life-cycle
• reduce the number of exceptions to ground rules of View.has_finished()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.65%. Comparing base (e61219b) to head (1c53845).
Report is 29 commits behind head on branch-3.5.

❗ Current head 1c53845 differs from pull request most recent head e3e1ba6. Consider uploading reports for the commit e3e1ba6 to get more accurate results

Additional details and impacted files

@@              Coverage Diff               @@
##           branch-3.5   #13801      +/-   ##
==============================================
+ Coverage       91.57%   92.65%   +1.07%     
==============================================
  Files             326      326              
  Lines           20737    20734       -3     
==============================================
+ Hits            18990    19211     +221     
+ Misses           1747     1523     -224