Refactoring

[jakobandersen]

While trying to figure out how things work together and fix an issue I got a little side-tracked, so:

• Add type-checking via mypy to the Github CI (make type-check).
• Add a lot of typing to much of the code.
  Notably not to the XML reading as it seems to be auto-generated.
• Remove some factories and simplify the related code.
• Simplify the initial setup code and make the core Sphinx application object directly available in more classes.
• Shuffle a few things around for structuring.

[vermeeren]

Sorry for the long wait and thanks as always.