TextArea: use TextEdit::append()

Change-Id: I30f23198cd82f26f92d3b7cae47e2ac64d1fad9b
Reviewed-by: Caroline Chao <caroline.chao@digia.com>

src/controls/TextArea.qml

@@ -412,9 +412,7 @@ ScrollView {
         Appends \a string as a new line to the end of the text area.
     */
     function append (string) {
-        if (length)
-            string = "\n" + string
-        text += string
+        edit.append(string)
         __verticalScrollBar.value = __verticalScrollBar.maximumValue
     }