add mapmodelcull / soundcull / decalcull

[Jigoku]

I believe this was mentioned previously as being useful, but couldn't see it on the tracker.

Add commands to remove unused assets from a maps config file, just as how texturecull / texturerehash work. But for mapmodels, sounds, and decals. This would be particularly useful to quickly know whether any maps in the default rotation are using a specific asset. For instance, if a mapmodel needs adjusting in some way, assuming that the maps would be culled of unused assets in the config file, you could easily grep the map configs to know whether or not it needs adjusting within those maps in the event changes are made to an asset.