spearmint: Merge PAKSUMS and gameconfig.txt to mint-GAMEDIR.settings

The new filename for mint game settings is mint-GAMEDIR.settings.
It is loaded from pk3 files, whereas PAKSUMS was not because I don't
know how to get file from a specific game directory.

Currently, only paksums are added from basegame settings files.

I originally wanted to put the extra settings in PAKSUMS, but instead I
added gameconfig.txt because it didn't fit the filename.

I removed support for PAKSUMS' "config noPakMismatchWarningDialog".