Send Threaded Messages

This MR deals with only sending threaded messages. Showing threads will turn up in a follow up. This allows you to start a new thread by clicking reply in thread to a normal message. 

You can also do a threaded reply to a threaded message in the main timeline at the moment because those messages aren't shown in a separate thread timeline yet but will be in future.

src/eventhandler.h

@@ -326,6 +326,20 @@ public:
      */
     QVariantMap getReplyMediaInfo() const;
 
+    /**
+     * @brief Whether the message is part of a thread.
+     *
+     * i.e. There is a rel_type of m.thread.
+     */
+    bool isThreaded() const;
+
+    /**
+     * @brief Return the Matrix ID of the thread's root message.
+     *
+     * Empty if this not part of a thread.
+     */
+    QString threadRoot() const;
+
     /**
      * @brief Return the latitude for the event.
      *