move timed asset prioritization/control to audiotrack from project

[hburgund]

Right now there is a boolean project.timed_asset_priority to determine if timed assets take precedence over simultaneously available geo-assets, but this is too simple to handle the variation I need.

If we move this to the track level, we can have some tracks behave differently from other tracks which will allow for much more nuanced control of the experience.

Additionally, this field needs to be an enum not a boolean because there are actually four states to manage, not just two.

• highest timed assets are prioritized above geo-assets on this track
• normal timed assets and concurrently available geo-assets are given the same priority resulting in a randomized mix of the two
• lowest timed assets will only play if no geo-assets are currently available
• discard this track plays no timed assets