Markers should use core.Item

Hi, I'm having issues with markers, I'm implementing a treeview representation of the otio structure, which is going well but Markers are a pain as they are not parented, which is odd, as they are useless out of context, they always need a parent for calculating their position in the timeline.

So would it make sense for then to work the same way as every other structure in otio, i.e. using core.Item, this also means that their clip/timeline position can be resolved through a function call, instead of guesswork (took ages to work out how they work).

cheers

The issue does not name any files, tests, or entry points. Begin by locating the marker and core.Item implementations, then review how parent context and timeline positions are resolved; done means an agreed design and corresponding behavior for markers.

Written by the indexing model from the issue text.