Change data structure for conflict resolutions

Now a dict keyed by (mod, defaultId) -> assignedId, starting at None
and then changed if there is an edit. Initialized to all None, then
conflicts are detected, and the 'resolutions' dict is populated with
the new IDs for each (mod, defaultId) that conflicts.
Prepopulate resolutions with 'preferred' IDs from NEI
Force mods to highest priority, if from 'preferred' IDs in NEI dump
Skip resolutions (from NEI dump) to same IDs. NEI dump import functional.
Document NEI dump support