opacityChanged is not called on child items of QuickItem

https://doc.qt.io/qt-6/qquickitem.html#opacity-prop specifies that opacity is propagated to child items, but opacityChanged is not triggered for child items so it is impossible to react to an opacitychange in child items.

You might want to do something in the child when opacity changed. This is impossible now since only the parent gets the message that the opacity changed.

1. Unpack 7zip
2. Go to QtBugReproducer
3. cd build
4. cmake.exe -A x64 -T v143 ..
5. start devenv.exe QtQMLGraphicsItems.sln
6. build project
7. call %QT_ROOT_64%\bin\windeployqt.exe --qmldir . build\Debug\QtQMLGraphicsItems.exe (from the source directory)
8. Start the executable
9. Use button to change opacity
10. Observe console ouptut
11. Change line 31 in main.qml
12. Rebuild
13. Start executable
14. Use button to change opacity
15. Observe console output

Expected:

      9) Both Rectancle 1 and 2 should print that opacity changed

     15) Both Rectangle 1 and 2 should print that opacity changed

Observed

      9) Only Rectangle 1 prints that opacity changed

     15) Both Rectangle 1 and  2 should print that opacity changed (but the code change in (11) should not have been needed.