Identifier clashes in DOM when multiple score containers contain renderings of the same MEI file

[musicog]

... and more generally, whenever different MEI files share identifier names.

Problem: The fragment URI's prefix up to the # is stripped in-application before rendering annotations. We should instead replace the prefix with the relevant score container's React key, and make a method for "getByOriginalID" for those containers.

[DILewis]

To clarify different levels of issue:

1. ID clashes: Lots of components turn IDs in source files (especially MEI and TEI) into DOM IDs. There is no guarantee that these will be unique, making the resulting app invalid HTML and it's functionality, potentially, unreliable.
2. URI reuse: The same file, rendered twice may have the same IDs duplicated because of 1., but also, depending on how component keys are generated, may have clashing react component keys. We provide no clear mechanism to distinguish between elements from multiple renderings of the same file within the app. This may have an impact on preserving or recreating state.
   Where it is important to be able to refer to state specifically, we will need react keys to be reproducible (rather than random). It's probably worth noting that we've got relatively little currently for saving and loading state anyway.

To be clear, there is no problem keeping track of URIs referring to external materials or parts of them within MELD, this is largely (entirely?) an issue of the DOM and of the application state.