API: Versioning policy for removing public APIs

Thus far apis.h only had instructions for adding APIs, not removing
them. Restored the old enums of the removed W API.
Fixed|libcommon: Linkage conflict (extern C function returning a C++ reference)
WadMapConverter: Id1MapElement needs a non-default copy-ctor
Merge branch 'master' into id1-import
libcore|PathTreeT: Added tryFind() specializations to PathTreeT
ResourceSystem: Locate id Tech 1 maps in loaded packages during game resource init