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/qml/MessageDelegate.qml

@@ -187,6 +187,10 @@ TimelineDelegate {
      */
     required property var replyMediaInfo
 
+    required property bool isThreaded
+
+    required property string threadRoot
+
     /**
      * @brief Whether this message is replying to another.
      */