Don't destroy formatting when editing previous messages

Adds a few new methods to grab the markdown/slightly rich text from the
message, and will intelligently re-insert user mentions as needed.

src/eventhandler.h

@@ -185,6 +185,13 @@ public:
      */
     QString getPlainBody(bool stripNewlines = false) const;
 
+    /**
+     * @brief Output the original body for the message content, useful for editing the original message.
+     *
+     * The event type must be a room message event.
+     */
+    QString getMarkdownBody() const;
+
     /**
      * @brief Output a generic string for the message content ready for display.
      *