CategoryTool_importCategoryFile: Refactor.

Factorise code paths between simulation and "real" modes,
fixing occasional non-representativeness of simulation mode.
Factorise code (down from 300 to 200 lines).
Simplify code (ex: use set instead of None-value dict).
Merge consecutive loops:
- delete/expire immediately instead of building a list to process later
- process each Base Category entirely (create, edit, delete/expire/keep)
before going to the next
This merging should overall reduce memory usage significantly.