299| Refactor: Frame management

Description of refactor

• Make get_raw_sequence fall back on the currently display Movie or Image sequence if nothing else is caught.
• Refactor drawing annotations with sequence.frame_ attributes.
  • sequence.frame_start is used to calculate frame in the media time.
  • sequence.frame_offset_start is used to set the frame offset.
  • sequence.frame_final_start is used to calculate frame in the timeline time and handle sequence positioning.
• Moved meta making of video + sound after the Aspect ratio preservation for ensuring sound offset and being able to rely on raw sequence frame_ attributes.
• raw_sequence renamed as media_sequence.

@olivier.hoareau I need you to test that on your side ASAP because I need it to continue my rebase of !268 (merged).

Also closes #297 (closed)

How to test it

Load a timeline with handles and offsets.

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: @strangenoise