[douglas]

Moddable stacking rules. For each ability that could conceivably be stackable, allow the specification of stacking groups. Wherever stacking groups are defined, possibly in their own file, allow independent specification for each ability value if an ability has multiple values. Both highest-only and lowest-only should be options, as well as infinite stacking. Also, limits should be possible for both the number of distinct source that can stack and the minimum/maximum total value after stacking. Stacking groups should also have a list of other stacking groups that they stack with. Stacking groups may or may not be specific to particular abilities. Which stacking group to use would be specified along with all the other ability information in components.txt, facility.txt, etc., and would be separately specifiable for each ability the item uses.