Remove the abstraction of dictionary format

This is the beginning of a larger refactoring needed for the live dictionary update feature. I believe this change removes some unnecessary complexity.

This change removes the concept of a dictionary format option. Plover will now use one format internally to represent steno strokes. It will still read both of the currently supported formats and convert on read to the internal one. In the future if we want to support other types of dictionaries then we will convert them on load.