The properties are still declared in the Python 3.5 compatible assignment
notation, and a class decorator that converts those to class annotations
as preferred by Blender 2.80.
This includes using `''` instead of `None` in some cases where an empty
string conveys 'nothing' equally well as `None`; in such cases keeping the
type the same rather than switching to another type is preferred.
The `file_loading` function is called deferred by asyncio, and can thus
be called when the operator has already stopped loading. This is fixed by
not referring to `self` in that function, and taking the logger from the
outer scope.
