add_value
Description:
Sets the GValue for a given tag using the specified mode.
Parameters:
| this | list to set tags in | 
| mode | the mode to use | 
| tag | tag | 
| value | GValue for this tag | 
Sets the GValue for a given tag using the specified mode.
| this | list to set tags in | 
| mode | the mode to use | 
| tag | tag | 
| value | GValue for this tag |