Get rid of "Fighter" class

[Daniel Kraft]

Currently, Fighter is essentially a manual polymorphism implementation (a wrapper around either Character or Building handles, exposing the combat-related things with a common interface).

Instead of that, we should just use the new CombatEntity superclass to access common stuff, and then use std::unique_ptr<CombatEntity> as the handle type. FighterTable should probably stay, so that we can query the union of buildings and characters easily.