'Pop up a dialog window for input of a string.
Arguments: title is the title of the dialog window,
prompt is a text mostly describing what information to input.
Return the string input
If the dialog is canceled, return None.
Example (for a TurtleScreen instance named screen):
>>> screen.textinput("NIM", "Name of first player:")'

def textinput(self, title, prompt):

    return simpledialog.askstring(title, prompt)
