messagedelegate: add author property to DataTransferMessageDelegate

Avoid potential undefined access to child author property

Change-Id: I3926243ff20cab9c66ac8c6e6b61e71fe46b8a63

src/commoncomponents/DataTransferMessageDelegate.qml

@@ -34,6 +34,7 @@ Loader {
     property var mediaInfo
     property bool showTime: false
     property int seq: MsgSeq.single
+    property string author: Author
 
     width: ListView.view ? ListView.view.width : 0