API: Added HOOK_GAME_INIT

Called when initializing a loaded game. This occurs once all startup
resources are loaded but before parsing of definitions and processing
game data. This is a suitable time for game data conversion.
F API: M_ReadFileIntoString() supports paths like "LumpIndex:<lumpnumber>"

The file to read can now be specified with a URI like reference into
the engine's central LumpIndex.
IdTech1Converter: Connect MAPINFO translation to HOOK_GAME_INIT

Parse and merge the content of all loaded MAPINFO lumps, in load order.

Todo: Output the translated DED definitions to a temporary file which
will be parsed automatically later, during game initialization.