Refactor the representation of a word inside Studio

[joanise]

Currently, in readalongs/align.py we use dicts to store words, typically with a start, end and then id or text. This is confusing and difficult to use and document correctly. See discussion starting with "Orthogonal to this PR" in #119

Implementation to consider: reimplement a Word class using @dataclass and have the .text attribute calculated (and cached) on use, so that we just have one Word object type with the different query attributes we need, thus making it at lot more intuitive to both document and use.