Kdenlive adapter: process recent project format

[vpinon]

Fixes #1159
Fixes #1160
Don't rely anymore on old "global_feed" attribute to identify main tractor, but check last tractor in xml is the top one.

[codecov-commenter]

Codecov Report

Merging #1191 (19b8dd2) into main (1e44ca2) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1191      +/-   ##
==========================================
- Coverage   86.08%   86.05%   -0.03%     
==========================================
  Files         191      191              
  Lines       19228    19234       +6     
  Branches     2292     2295       +3     
==========================================
  Hits        16552    16552              
- Misses       2123     2128       +5     
- Partials      553      554       +1     

Flag	Coverage Δ
unittests	86.05% <0.00%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
...ontrib/opentimelineio_contrib/adapters/kdenlive.py	80.37% <0.00%> (-1.87%)	⬇️

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[jminor]

Thanks @vpinon and @codingcatgirl!