Fix includes order and add the needed ones to avoid transitive includes.
Remove QT_{BEGIN,END}_NAMESPACE because these are private Qt macros that
should not be used in the examples.
Task-number: QTBUG-117147
Pick-to: 6.6 6.5
Change-Id: I6caa0a59f2ab4dfec6fb558a0896c27b05b6a5db
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>