TableView: Make "model" property consistent with ListView

[ChangeLog][QtQuickControls][TableView] "model" property exposed
to delegates is now consistent with ListView behavior.

task-number: QTBUG-33107
Change-Id: I8315c32a9d8beda9901ed65b552f0b8a587316ee
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>