Improve the internal NodeVisitorEx API

Allow the implementation of visit to control whether the children should be
processed afterwards or not. This is modelled after the JS AST visitor in
Qml.

Change-Id: I5b1adf5e1aefd8899bf444e993ddfbf2c5c125c9
Reviewed-by: Lars Knoll <lars.knoll@digia.com>