RV Adapter Marker support

[ssteinbach]

I think Markers would become "marks" in RV. Having them come through would be cool in the RV adapter.

[ssteinbach]

Current implementation of Marks is limited compared to what markers can do in OTIO - I don't think its worth us spending too much time on this right now. Limitations include:

• Global list of frames in a session (not per-sequence as far as I can tell, might be misunderstanding this)
• No metadata per marker (that is inspectable in the UI in anyway)
• Single frame, no ranges

[meshula]

RV support is now native, not a plugin maintained here.