Don't draw anything in TableView unless there is a column

I was really confused because after assigning a model it looked like
the table view was showing the data, while in reality it was
just filling up with alternating row colors. I think it is
better not to draw anything to ensure that people correctly add
a TableViewColumn.

Change-Id: I353b478f4af80410aca3f94439a02ed7ab53f6d6
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>

src/controls/TableView.qml

@@ -304,8 +304,9 @@ ScrollView {
         anchors.topMargin: tableHeader.height
         anchors.fill: parent
         currentIndex: -1
-
+        visible: columns.length > 0
         interactive: false
+
         SystemPalette {
             id: palette
             colorGroup: enabled ? SystemPalette.Active : SystemPalette.Disabled