Fix N^2 (over items) performance in otioview _populate

[ssteinbach]

• We added the range_of_all_children method for exactly this use case- it was missing here
• This significantly improves performance of reading large files into otioview

[codecov-io]

Codecov Report

Merging #422 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #422   +/-   ##
=======================================
  Coverage   86.85%   86.85%           
=======================================
  Files          63       63           
  Lines        5608     5608           
=======================================
  Hits         4871     4871           
  Misses        737      737

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