The code was too much spread out, in core and tool code, and also it was
made too specific to fill. I'll want to reuse this code at least in the
fuzzy select tool. This will avoid code duplication, and also make this
new process more self-contained and simpler to review later (the
algorithm also has a lot of settings and it is much cleaner to have them
as properties rather than passing these as parameters through many
functions).

The refactoring may not be finished; that's at least a first step.

(cherry picked from commit db18c679)