Changed the Calculator class to implement DocumentListener and its associated method stubs, because that is how JTextFields communicate their changes.
Revert "Changed the Calculator class to implement DocumentListener and its associated method stubs, because that is how JTextFields communicate their changes."

This reverts commit 7b48d91a5df60d98ba12c727cf1635ab8063edd2.
Merge branch 'master' of https://github.com/HisModusness/Dynamic-Calculator