Fixing crash related to Undo

[jonoomph]

Related to OpenShot/openshot-qt#4965

• New unit tests to verify Readers and FrameMappers are correctly cleaned up
• Some refactoring to ensure no double free on Reader cleanup

[jonoomph]

Improve DeepCopy Performance

[codecov]

Codecov Report

Merging #864 (1547fb1) into develop (1186c24) will increase coverage by 0.60%.
The diff coverage is 95.18%.

❗ Current head 1547fb1 differs from pull request most recent head 7a733c0. Consider uploading reports for the commit 7a733c0 to get more accurate results

@@             Coverage Diff             @@
##           develop     #864      +/-   ##
===========================================
+ Coverage    50.48%   51.08%   +0.60%     
===========================================
  Files          184      184              
  Lines        15994    16063      +69     
===========================================
+ Hits          8074     8206     +132     
+ Misses        7920     7857      -63     

Impacted Files	Coverage Δ
src/Timeline.cpp	55.23% <63.63%> (+5.78%)	⬆️
src/Clip.cpp	59.09% <100.00%> (+2.53%)	⬆️
tests/Clip.cpp	100.00% <100.00%> (ø)
tests/Timeline.cpp	100.00% <100.00%> (ø)
src/FrameMapper.cpp	94.69% <0.00%> (+0.66%)	⬆️
src/Timeline.h	82.60% <0.00%> (+4.34%)	⬆️
src/DummyReader.h	100.00% <0.00%> (+33.33%)	⬆️
src/QtImageReader.h	100.00% <0.00%> (+33.33%)	⬆️

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