OTIOView should show media references

[ssteinbach]

We're starting to use Clips with generator references internally, and it occured to me that identifying clips with External References in some way that is different from clips with different kinds of media references could be helpful.

[jminor]

I'm hijacking this issue (renaming it) to encompass a slightly larger goal of having OTIOView show media references clearly. It should perhaps color code or badge them based on type, show how available_range and source_range align, overlap, or conflict, and should show multiple media references when they are available.