Fix crash when object children iterator is invalidated

[MelodicsPavol]

Calling QQmlContext::nameForObject can have side effects, creating new child objects via property read functions.

Reproduction steps:

1. in examples/Basic/main.qml change Window to ApplicationWindow
2. in examples/Basic/main.cpp MyTests::executeTest() add a line std::cout << existsAndVisible(spix::ItemPath("mainWindow/Button_X"));

ApplicationWindow has screen property, accessing which adds a new QQuickScreenInfo object into children list. If this happens during list iteration in range loop, the iterator will be invalidated. Using index based for loop will avoid crash scenario.

Reproducible with Qt 6.2 LTS.

[walterbrebels]

Spix server crashes due to this issue, is this fix ok to merge?

[faaxm]

Thank you @MelodicsPavol !