Add an analogue of damageterraineffect in terrain definition

[Kzer-Za]

It seems that in UDMF map format you can specify damageterraineffect boolean property for a sector, which means that it should spawn a terrain splash when damage is inflicted.

If it's not too hard to implement, it would be nice if an analogue of this variable was also available in the terrain definition.

Rationale:
Currently a terrain splash is not spawned when damage is inflicted by terrain. Which means that if you want such splashes, you have to modify each sector manually. (Or deal damage not via terrain definition but via sector special, which also means assigning a sector special to each sector manually.)

It would be nice if the engine spawned a terrain splash upon terrain dealing damage, if damageterraineffect OR that new terrain variable was set to true.

If a terrain definition had such a variable, it would save a lot of manual editing of maps; and if it by default was set to false, it would not break any existing maps.