set_tooltip_window
Description:
Replaces the default window used for displaying tooltips with custom_window.
 GTK+ will take care of showing and hiding custom_window at the right moment, to behave likewise as the default tooltip window. If
       custom_window is null, the default tooltip window will be used.
Parameters:
| this | 
           a Widget  | 
      
| custom_window | 
           a Window, or null  |