Implement fire created by explosions

[michaeldgg2]

For example when unit is destroyed, fire should start.

• [ ] visual effect (fire)
• [ ] visual effect (smoke)
• [ ] visual effect (light)
• [ ] spread damage over time
• [ ] burnt tile underneath
• [ ] sound effect (fire burning)

Use custom CellLayer to implement management of fire.

[michal-mad]

Actor idea sounds more feasible.

• Can use infantry subcells
• Custom SpawnActorWarhead exists
• Spawned fire can belong to creeps, so if there is anything standing on it, it will be doing "melee" damage.
• fire actor can easily not occupy space
• could use AmbientSound for fire sound (and modify it so the sounds do not add up)
• Could use particles emitter for smoke.
• The fire size could be controlled by fire's HP.