Add "Feature Request..." entry and body templates in both types of issue

New entry under Help menu for requesting issues with the same template
used for a New Issue inside GitHub.

The "Bug Report..." entry has also been updated for adding the
corresponding template body.

Separators added in Help menu, since there are currently several different
options.

See related issue #1497

src/MainWindow.h

@@ -269,6 +269,7 @@ private slots:
     void checkNewVersion(const QString& versionstring, const QString& url);
     void on_actionWiki_triggered();
     void on_actionBug_report_triggered();
+    void on_actionFeature_Request_triggered();
     void on_actionSqlCipherFaq_triggered();
     void on_actionWebsite_triggered();
     void on_actionDonatePatreon_triggered();