js: update getDefaultKey to more precise logic

--

* src/Keyring.js: Adapted Keyring.getDefaultKey() to my current
  understanding  of a default signing key: either the default key set
  in the gpg config, or 'the first usable private key' - usability
  meaning  'not invalid, expired, revoked, and can be used for
  signing'. It should be the same key used as in command line when
  doing a --sign operation.
  In case the user has a smartcard plugged in, we currently
  won't know of this here, so our choice may differ. But as we do all
  javascript-binding sign operations with the key  fingerprint
  explicitly set, this should not be a real problem. This method is
  seen more as a convenience to tell using librarys which key
  represents the main user.

lang/js/src/Keyring.js

@@ -221,7 +221,12 @@ export class GPGME_Keyring {
                             reject(gpgme_error('KEY_NO_DEFAULT'));
                         } else {
                             for (let i=0; i< result.keys.length; i++ ) {
-                                if (result.keys[i].invalid === false) {
+                                if (
+                                    result.keys[i].invalid === false &&
+                                    result.keys[i].expired === false &&
+                                    result.keys[i].revoked === false &&
+                                    result.keys[i].can_sign === true
+                                ) {
                                     let k = createKey(
                                         result.keys[i].fingerprint,
                                         !prepare_sync,