Implement 100% enemy counter

[chinese-soup]

Including a functionality, where infinite monstermakers are either not counted towards the 100% or are made to be one-time only.

[YaLTeR]

I had something like this in mind:

• There's a killed / total counter, both of those numbers update as you kill monsters and as new ones spawn (e.g. you go to the next map) respectively.
• The counter behaves as expected with regards to saving and loading.
• Regular monsters add to total on spawn and to killed on dying.
• One-time spawners:
  • If we decide that not triggering the spawner is a valid 100% strategy, they don't do anything special, spawning mobs are handled by the rule above.
  • If we decide that for 100% you need to trigger all spawners, they can add to total their mob count right as the spawner itself appears on the map.
• Infinite spawners: behave similar to one-time, but subsequent mob spawns don't contribute to total and their kills don't contribute to kill.
• If there are some ill-behaved spawners (like with a 1 hour delay or something), we might need a manual override system to filter them out.