293|Bugfix: Sequencer update issue when otio timeline changed

Description of fix

scene.tracks.clear() when source timeline is different from cached one. This is not the most optimized way to do it, but it's better to be reliable.

Speed will be fixed with #258 (closed)

How to test it

• Load a timeline.
• Change the source .otio timeline.
• Reload the same timeline.

Checklist

Please check if your MR fulfills the following requirements:

• Build was run locally and any changes were pushed
• Lint has passed locally without error
• CHANGELOG.md has been updated
• Branch has been rebased on merge branch

FYI: @olivier.hoareau