Fix new thumbnail default layer from Hiero 16.0

[rdelillo]

Changelog Description

Foundry has changed the default layer for the thumbnail API:

• used to be colour (https://learn.foundry.com/hiero/developers/14.0/HieroPythonDevGuide/api/api_core.html?highlight=layer#hiero.core.SequenceBase.thumbnail)
• it is now rgb (https://learn.foundry.com/hiero/developers/latest/HieroPythonDevGuide/api/api_core.html?highlight=layer#hiero.core.SequenceBase.thumbnail)

The layer thing is something undocumented from the hiero API. Also during my tests I found out clips that contained neither "rgb" or "colour" layers. Not sure how this is possible, but I made sure those track items also handled properly.

Testing notes:

1. Open new sequence from Hiero 16.0+
2. Ensure new track items thumbnail get extracted from new sequence
3. Ensure new track items thumbnail get extracted from old sequence
4. Ensure incompatible track items are reported with a graceful PublishError

closes #98