Add proper cOverworld constructor so a COMPLETE object is created.

For further explanations, see issue #17.
Add XML loader for the overworld description files
Fix order of statements in overworld description loader
Add FIXME to level.cpp
Parse cOverworld_description XML in cOverworld constructor if ENABLE_NEW_LOADER is enabled.

Also note that cOverworld instances don’t refer to a single file
with `m_path' as was incorrectly described in the comments before
this commit — a SMC world consists of a directory containg three XML
files (one of those is the description XML file we can now parse
with this commit).