Add spawnCondition field to all entities

[XenonA7]

Krypek discovered that spawnCondition is a global property that all entities can have. The map editor already supports spawnCondition for various entities like props because spawnCondition is a defined attribute in the code of ig.ENTITY.Prop.

However, something like ig.ENTITY.Door does not have it as an attribute and thus the map editor doesn't let you define it. Yet you can add spawnCondition to a Door entity using json and it works like any other entity with spawnCondition.

I needed to use this for a door recently, it would help if the map editor let you define spawnCondition for all entities.