OTIOView should show media references

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.

Start by locating the OTIOView entry point and reading how clips with media references are currently represented. Clarify which visual distinction is required for External References versus other media references, then define completion around OTIOView showing that distinction and add or update the relevant tests if the project has them.

Written by the indexing model from the issue text.