added steam directory config entry

New value for Steam directory in config.txt used if set

Uses Steam directory set in config.txt if set

Changed Steam Directory section to Steam and Steam parameter to Directory

Added config_steam_location to get Steam's directory from the config file, and changed the wording of some of the error messages to be more instructive.

Asks the user for Steam's userdata directory when not found

fixed spelling error in comment

Changed Steam Directory section to Steam and Steam parameter to Directory