Start parsing the overworld XML file.

Note the cOverworld_Loader class now builds up a (mostly) complete
cOverworld instance, but it still is not complete. Therefore the
entire XML loader class should be considered a private API not to
be used externally. Use the cOverworld constructor to create a
complete cOverworld instance.
Parse settings XML tag
Start SMC exception hierarchy
Add XmlAttributes::retrieve() for type-fetching keys without default values.
Parse Player XML tag
Parse XML tag Overworld
Start parsing the object XML tags in overworlds
Parse sprite XML tags in overworlds
Parse Waypoint XML tag
Change cOverworld constructor to static member.

Otherwise we would have to copy all the attributes over from the
result of the world XML parser (which already _is_ a cOverworld
instance).