Deleting key automatically removes it from agent.
Renamed KeyModel's fingerprintPrivateKeyFile method to
getFingerprintAndKeyTypeFromPrivateKey, and made it set
KeyModel's self.publicKeyFingerprint if it finds the key
in the agent.  That way, we can check if self.publicKeyFingerprint
is not None in removeKeyFromAgent, instead of bailing out if
self.pubKey is None.  self.pubKey will only be set if we have
added the key in this current Launcher session, i.e. we can't
use the self.pubKey!=None method to check whether it's OK to
run removeKeyFromAgent if we have exited and relaunched the
Launcher, and then we want to remove the key using the
Identity menu.