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.

(cherry picked from commit e2eb6ab33c)

src/eventhandler.h

@@ -159,6 +159,14 @@ public:
      */
     bool isHidden();
 
+    /**
+     * @brief Output a string for the room message content without any formatting.
+     *
+     * This is the content of the formatted_body key if present or the body key if
+     * not.
+     */
+    static QString rawMessageBody(const Quotient::RoomMessageEvent &event);
+
     /**
      * @brief Output a string for the message content ready for display in a rich text field.
      *
@@ -191,6 +199,13 @@ 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.
      *