MediaType-defined FrameLevel custom attributes

[sbatchelder]

Localizations often have attributes that are shared between all Localizations from the same media frame, regardless of version. For example Depth, and other water-properties. Instead of assigning these values to all Localizations, it should be possible to associate metadata (custom attributes) to all the frames of a media without having to create a Frame-level State for all frames of all given MediaTypes (Doing so with the existing frame-level States method is impractical using the current UI). Regular frame-level states are still useful for tracking other attributes and being version-dependent, just not ones that span all frames of the video for frame metadata.

MediaTypes should have a special set of frame-level attributes. The attributes can be populated by an uploaded media-metadata file or initialized to defaults and updated using the api and be version-independent. These would be attributes that all frames of a media of a given mediatype would have, and Localization would be able to be filtered by these attributes, or inherit/have access to these attributes when downloaded, perhaps via a "download with frame metadata attributes" boolean flag.