Refactor|libdoomsday|LumpIndex: Implement pimpl in LumpIndex using DENG2_PRIVATE, cleanup
Refactor|libdoomsday|LumpIndex: Added LumpIndex::findAll(), renamed other members for consistency

LumpIndex::findAll() collates the indices (lumpnum_t) of all lumps
in the index whose path matches the search term (in load order).
Cleanup: Use LumpIndex::operator [] (lumpnum_t)
ResourceSystem|CompositeTexture: Dispose of PatchNames once CompositeTextures are initialized, cleanup
Cleanup: Use LumpIndex::findAll() for processing DD_DIREC/DD_DEFNS lumps

Also moved printing of the lump index contents out of LumpIndex.