Add support for new Clip Color schema

[timlehr]

Since OTIO gained an official Color schema (see AcademySoftwareFoundation/OpenTimelineIO#1887), we should update Raven to use it instead of relying on it's own color metadata. Currently the color value has no effect on the app. This also needs to be added for the editing capability.

    "OTIO_SCHEMA": "Clip.2",
    "color": null,
    "metadata": {
        },
        "raven": {
            "color": "YELLOW"
        }
    },