Associates a new value with key under group_name.
If key cannot be found then it is created. If group_name cannot be found then it is created. To set an UTF-8
string which may contain characters that need escaping (such as newlines or spaces), use
set_string.
| this |
a KeyFile |
| group_name |
a group name |
| key |
a key |
| value |
a string |