Issue #19: Make creeps ignore moveable props

Creeps now ignore any prop_physics objects that are moveable.
Notes:
- They can kill themselves on props, but it doesn't happen much,
- Verified AI still "sees" doors, etc.
- This was added to a prop getting spawned, so should cover
both map-defined and "summoned" props. (Not verified).
- Uses svar sv_dotaCreepsIgnoreProps (Default: 1)