Improve TableView performance by caching row-delegates

This gives a much better scrolling performance in TableView
since we do not have to instantiate the rows or re-establish
bindings to header columns for each created table row.

[ChangeLog][QtQuickControls] Significantly optimized TableView
scrolling by recycling rowDelegate instances.

Change-Id: I301e5e476c78f241be1282332905b714fbc5b37d
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>