Simplify the usage of datafile reader and writer by adding utility functions to read and write zero-terminated UTF-8 strings.
Improve validation of string data read from datafiles. It is ensure that string data is null-terminated, has no internal NUL-characters and is valid UTF-8.
Fix loading of external sounds in the editor. The wrong path variable was being used, so the sound files would not be loaded from correct folder.
Add tests for new datafile reader/writer functions.