QmlDesigner: Refactor NavigatorTreeModel::updateItemRowOrder

Change-Id: I0b89f81cb122704b827395c937a0b7dd7d605722
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner: Refactor appendNodeToEndOfTheRow

Change-Id: I7d4c26ce118e6346be7110002cc2197970392aa4
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner: Change NavigatorRole in enum

and change name to InternalIdRole

Change-Id: I130de99e02152eab8b71a1a2ab86899acc30b2c6
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Marco Bubke <marco.bubke@digia.com>