Minor improvements

- Added a key in the configuration file 'ExtraMixes' that allows for
loading additional MIX files to be loaded
- Added a log message for trying to use a config (AKA mod/game) that is
for TS or FS
- Added a log message for MIX files that cannot be loaded at all (last
attempt is done in MixManager)
- Made output.log and debug.log use the exact same time string that's
used for the filename