Get rid of std::string inside screentext.cpp in favor of char* wrapper

This addresses the current Synthesis issue where release builds are failing, and is related to #134 (closed).

@hendricks266 mentioned that he wanted to get rid of std::string inside screentext.cpp anyways. Since it's been a while since I debugged and reported the issue, I assume he doesn't really have the time to do this at the moment, so I made an attempt myself.

I tested it with a localization DEF file, which I have attached to this merge request. Simply type in the console "locale de" and open the cheat entry menu. If it changes to German, the localization is still functional: duke3d.def

I marked this as a WIP because I'm not sure if this is an acceptable solution to the problem, and would like some feedback.