set_text_with_mnemonic
Description:
Sets the label’s text from the string str.
 If characters in str are preceded by an underscore, they are underlined indicating that they represent a keyboard accelerator 
      called a mnemonic. The mnemonic key can be used to activate another widget, chosen automatically, or explicitly using 
      set_mnemonic_widget.
Parameters:
| this | 
           a Label  | 
      
| str | 
           a string  |