TableView: fix a performance issue with row delegate instantiation

To avoid accidentally instantiating all rows at construction time, make
sure the rows have a valid (but arbitrary) height meanwhile the row
delegates haven't yet been loaded.

Change-Id: I1afc6bfa920886260717b3cb172fa0d858c0a8d5
Reviewed-by: Caroline Chao <caroline.chao@digia.com>

src/controls/TableView.qml

@@ -450,6 +450,7 @@ ScrollView {
                 sourceComponent: root.rowDelegate
                 // Row fills the view width regardless of item size
                 // But scrollbar should not adjust to it
+                height: item ? item.height : 16
                 width: parent.width + __scroller.horizontalScrollBar.width
                 x: flickableItem.contentX