Porting to Windows (which doesn't like pexpect).
Dialog now hides irrelevant fields, depending on radio button selection.
spelling
Fixed typo
Change passphrase dialog now uses a separate PrivateKeyModel class
to change the passphrase.

Deleting a private key now also deletes the corresponding (local)
public key file if it exists.
Change passphrase dialog now uses a separate PrivateKeyModel class to
change the passphrase.

DeleteKey now deletes the (local) public key file as well as the
private key file if it exists.
Removed PrivateKey.py, which has been replaced by PrivateKeyModel.py
DeleteKey class has been removed, and its functionality
has been incorporated into the PrivateKeyModel class.