Support OTIO-style markers in the OpenRV UI

[rogernelson]

OTIO-style markers contain some features that OpenRV's markers do not -- like range and colour. In addition, markers can be set on almost any OTIO schema (track, clip, etc) whereas in OpenRV they can only be applied on the global timeline level. During the feature implementation of OTIO support, properties were added to support these and set during OTIO Import.

The UI implementation, however, was never completed. It would be nice to be able to be able to show these more versatile markers in the UI.