Sync to DNEG internal repo

[tedwaine]

Sync to DNEG internal repo

This PR merges all recent work from DNEG's xSTUDIO development team on our internal repo, staging for v1.1.0 release.

New features available with this update:

• Video output feature.
  • Render playlists, contact sheets and timelines to a movie file.
  • Apply any FFmpeg encoding options that you require.
  • Overlay graphics and colour management will match xSTUDIO's viewport.
  • Renders run in the background, allowing you to continue working in xSTUDIO
• Set Media FPS
  • Set frame rate of individual media items, overriding their 'natrual' rate if desired
  • Slow down or speed up clip within timeline edits or in a playlist
• Zoom to Loop Region
  • Automatically zoom the time slider to the Timeline's loop region
• Notes
  • Note Subject/Title is now user editable

Some improvements to the Python API have also been added following community feedback. For example, you can now override the MediaReference of a MediaSource and thus switch the on-disk source 'in place'. There are also new options for python plugins to execute callbacks in response to UI interactions and for ways to execute methods in a Python plugin from the C++ core.

We have also fixed a bug in the Audio buffer cache that was causing occasional instability.

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