set_value
Description:
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.
Parameters:
| this | 
           a KeyFile  | 
      
| group_name | 
           a group name  | 
      
| key | 
           a key  | 
      
| value | 
           a string  |