NMACertChooser: On selection of a new certificate, clear the existing key
In the context of using a pcks12 certificate envelope (for both certificate and key), if the user selects a different certificate; then also clear the selected key and password, re-sensitize the key-input and remove any widget errors.
To avoid ambiguity, exhibit the same behavior when using .pem certificates. Select a different .pem certificate will imply also clearing the key, and password input.