Parse multi-text layers

[NickLaiacona]

Presently, each layer in a document is represented by a single text element and the xml:id of that element. However, another common use case is to have text layers for each page or section of a text. This is useful for editing manuscripts and working with larger texts.

To support this mode, we add an additional syntax to the xml:ids. If an xml:id has an '_' in it, the text after then last underscore is considered to be the layer ID. This allows multiple text elements to be designated as being part of the same layer, from EditionCrafter's perspective.