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

GpgMECWorker.h

@@ -31,20 +31,6 @@ public:
      */
     bool DeleteKey(const char * fpr, bool secret, GpgME::Error& e);
 
-#ifdef DEVTIME
-    /**
-     * Status : testing
-     * \n Result : fails to export a private key
-     * \n Reason : passphrase callback 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
-     * @return 
-     */
-    bool ExportPrivateKey(const char* fpr, GpgME::Error& e);
-#endif
-
 private:
     gpgme_ctx_t c_ctx;
 };