Allow exporting secret keys.

Requests the passphrase with a popup.

As from GPGME 1.15.0, the loopback pinentry is functional when exporting secret keys. It works fine when the exact passphrase is provided. If it's a wrong passphrase, GPGME does not generate an ::Error, but the app crashes with 'free(): double free detected in tcache 2'.

Hence, this patch cannot be committed to master.

Status : dangerous
Result : works and works not
Reason : a wrong passphrase means a crash

GpgMEWorker.h

@@ -162,21 +162,14 @@ public:
                              const char * algo,
                              const string& passphrase,
                              ulong expires = 63072000);
-
-#ifdef DEVTIME
     /**
-     * Status : testing
-     * \n Result : fails to export a private key
-     * \n Reason : loopback passphrase provider is never called
-     * \n With default pinentry mode, the password is requested normally 
-     * and the private key is exported. But this can't be done on a web server.
-     * @param fpr
-     * @param e
+     * Export a secret key.
+     * @param pattern : a key fingerprint
+     * @param buffer : returns data in armor mode
      * @return 
      */
     const Error ExportPrivateKey(const char * pattern, string& buffer,
                                  const string& passphrase = "");
-#endif
     /**
      * Export a public key.
      * @param pattern : a key fingerprint