Buffs and debuffs as a builtin feature

[rversteegen]

If buffs and debuffs were a builtin feature, attacks could directly modify multiple stats at once in a way that wears off after a while, wears off gradually, changes graphics or animations, stack properly with each other or prohibit stacking, and more. They would be simpler to use than attacks which target stats and then chain to a later attack to revert the change.

This proposal is at https://rpg.hamsterrepublic.com/ohrrpgce/Plan_for_stat_buffs
James wrote that plan up nearly 12 years ago but amazingly it hasn't dated at all. There's almost nothing in that proposal that I would change.

See also #994 for making buffs/debuffs change the affected's appearance, play animations, or show icons.

Equipment essentially acts as buffs: they change stats and elemental resistances. We basically already have code for applying and removing buffs: it takes as input a list of equipment, where each item of equipment adds one buff to a hero. We can ignore or put off until later the part in the Plan about adding a buff editor and converting equip to separately-defined buffs, and instead just duplicate part of the item data/editor into the attack data/editor. Then add a buff priority value so attack buffs are applied after equip buffs, and add a per-buff setting for how the buff acts (set values, add, multiply, etc).