Refactor: Consolidated and cleaned up methods of querying map author/title info

Map author and title info is now primarily available through read-only
console variables 'map-author' and 'map-name'. The old app variables
DD_MAP_NAME and DD_MAP_TITLE have been removed (obsolete).

Note that it is currently the games' responsibility to update these
accordingly, when the current map changes (because the engine has no
knowledge of Hexen's MAPINFO lump).

Further improved 'inspectmap' output to include the author and source
file path (if loaded from an add-on).