Refactor: load timeline function

Description of refactor

load_timeline function moved into utils_timeline to be used by linker coders independently from LoadTimeline operator.

How to test it

• Use stax.utils.utils_timeline.load_timeline(timeline_path) to load a timeline into Stax instead of the bpy.ops.sequencer.load_timeline(timeline_path="/path/to/timeline.otio") operator.

Checklist

Please check if your MR fulfills the following requirements:

• Unit tests have been adapted
• Build was run locally and any changes were pushed
• Lint has passed locally without error
• CHANGELOG.md has been updated

FYI: @olivier.hoareau