it is now possible to have multiple map models loaded at once - removed

it is now possible to have multiple map models loaded at once - removed
the isworldmodel flag, and submodel searches now use the mapname as a
second search key so submodels from multiple maps can coexist in memory
this is a cleanup that I had wanted to do for a long time...
added special modeldecompile support for submodels, they should now save
to a name based on the current map as well as the submodel number

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@8854 d7cf8633-e32d-0410-b094-e92efae38249