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

K7Main.cpp

@@ -142,7 +142,6 @@ K7Main::Create()
     vblButtons->addStretch(1);
     // Everyone can export a key
     m_btnExport = new WPushButton(TR("Export"));
-    m_btnExport->setToolTip(TR("TTTExport"));
     m_btnExport->hide();
     vblButtons->addWidget(unique_ptr<WPushButton> (m_btnExport));
     if (m_config->CanCreateKeys())
@@ -314,7 +313,8 @@ void K7Main::OnKeyAnchorClicked(WAnchor * source)
     DisplaySubKeys(id, secret);
     if (m_config->CanDelete()) // m_btnDelete is NULL otherwise
         m_btnDelete->setHidden(!m_keyringIO->CanKeyBeDeleted(id));
-
+    
+    m_btnExport->show();
     m_keyringIO->PrepareExport(id, secret);
 }