common: re-write get_environment_variable() without fixed buffer size
common: implement get_environment_variable() for non-Windows
GUI: don't display the preview warning if NO_WARNING is set to 1
GUI: automatically set the title from added source files
GUI: implement automatically setting the output file name
GUI: use .isEmpty() for stating intentions clearly
GUI: use .clear() for clearing lists

.empty() is just a compatibility function for stdlib and doesn't clear
the list. It's the equivalent of .isEmpty().

This fixes clearing the lists when loading/resetting configs.