Read enabled .mod files from ini file

[matthiakl]

Currently all files in Data/ and Data/modvdf are loaded. This prevents having multiple mods installed. With this change, only the .mod files listed in the ini file (with -game:<inifile>) are loaded.

[FILES]
vdf=FunnyMod.mod (Gothic 1)
VDF=FunnyMod.mod (Gothic 2)
List of all .mod files created for the mod. If this is more than one, they are listed separated by spaces (FunnyMod1.mod FunnyMod2.mod ...). GothicStarter first copies all *.mod files from .../Data/ to .../Data/modvdf/ before starting a modification and then copies all specified files from .../Data/modvdf/ to .../Data. The value "VDF" must exist (even if it is empty) for GothicStarter to accept the INI. In G2, the special value "!" can be used to explicitly disable reading by the GothicStarter. During development a VDF list is most useful. It is very important with G2 that the parameter GohticGame.mod is not here! This leads to the fact that all changes to the scripts are ignored.

https://wiki.worldofgothic.de/doku.php?id=ini-datei

[matthiakl]

https://ci.appveyor.com/project/Try/opengothic/builds/44531279/job/k1k02xtv7qyil1hc#L1688
Any idea why this fails to build on macos?

[Try]

In error log:

instantiation of undefined template 'std::ctype<char16_t>'

It seems MacOS doesn't support ctype for anything else than a char. so using std::getline is not an option. Generally it woruld be nice to avoid basic_stringstream, completely

[Try]

Merged, thanks for PR!