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

PopupExportSecretKey.h

@@ -0,0 +1,64 @@
+/* 
+ * File:   PopupExportSecretKey.h
+ * Author: SET - nmset@yandex.com
+ * License : GPL v2
+ * Copyright SET - © 2019
+ *
+ * Created on November 19, 2020, 9:01 PM
+ */
+
+#ifndef POPUPEXPORTSECRETKEY_H
+#define POPUPEXPORTSECRETKEY_H
+
+#include <Wt/WPopupWidget.h>
+#include <Wt/WContainerWidget.h>
+#include <Wt/WLineEdit.h>
+#include <Wt/WPushButton.h>
+#include "TransientMessageWidget.h"
+
+using namespace Wt;
+
+class PopupExportSecretKey : public WPopupWidget
+{
+public:
+    PopupExportSecretKey(WWidget * anchorWidget,
+                         TransientMessageWidget * txtMessage,
+                         const WLength& width = 300);
+    virtual ~PopupExportSecretKey();
+    void Create();
+    /**
+     * Used to forward the passphrase to the loopback passphrase provider.
+     * @return 
+     */
+    const string GetPassphrase()
+    {
+        return m_lePassphrase->text().toUTF8();
+    }
+    /**
+     * Caller binds its function here to add a link to the apply button.
+     * @return 
+     */
+    WPushButton* GetPreApplyButton()
+    {
+        return m_btnPreApply;
+    }
+    /**
+     * Expects a link with a shared resource that generates the secret key
+     * and forwards it to the browser.
+     * @return 
+     */
+    WPushButton* GetApplyButton()
+    {
+        return m_btnApply;
+    }
+private:
+    TransientMessageWidget * m_tmwMessage;
+    WContainerWidget * m_cwMain;
+    WLineEdit * m_lePassphrase;
+    WPushButton * m_btnPreApply;
+    WPushButton * m_btnApply;
+    WText * m_lblPassphrase;
+};
+
+#endif /* POPUPEXPORTSECRETKEY_H */
+