pass msg ID to single msg layout

resources/skins/plain/README

@@ -20,6 +20,7 @@ Here is overview of all placeholders used throughtout the skin:
     %5 - Date/time of the article.
     %6 - Attachments (composed from html_enclosure_every.html).
     %7 - Attached pictures (composed from html_enclosure_image.html).
+    %8 - Database ID of the message.
 
   html_adblocked.html:
     %1 - Title of the page. This page is used when the website is entirely blocked by AdBlock.

src/librssguard/gui/webviewer.cpp

@@ -154,7 +154,8 @@ void WebViewer::loadMessages(const QList<Message>& messages, RootItem* root) {
                                 message.m_contents,
                                 msg_date,
                                 enclosures,
-                                enclosure_images));
+                                enclosure_images,
+                                QString::number(message.m_id)));
   }
 
   m_root = root;