Added "No Value" option to all the combo box and also to the datetime UI elements; in case when the user does not set any value, default "No Value" will be selected which prohibits the auto selection while switching between the items.
Added a clear button to clear all the edited UI elements, especially required for clearing the combo-box and datetime elements.
Other minor refactorings: extracted the add_delete_button and add_clear_button as utility functions for common usage.