Second task of The 7 Tasks from 7GUIs

Challenges: bidirectional data flow, user-provided text input.

The task is to build a frame containing two textfields TC and TF representing the temperature in Celsius and Fahrenheit, respectively. Initially, both TC and TF are empty. When the user enters a numerical value into TC the corresponding value in TF is automatically updated and vice versa. When the user enters a non-numerical string into TC the value in TF is not updated and vice versa. The formula for converting a temperature C in Celsius into a temperature F in Fahrenheit is C = (F - 32) * (5/9) and the dual direction is F = C * (9/5) + 32.