kcm: Fix wrong settings shown when reverting changes
When reverting display changes, due to model changes, the StackLayout's currentIndex is changed by the stackLayout. This is a behaviour added in Qt 6.5, and requires working around to ensure we keep it the same as selectedOutput.
FIXED-IN: 6.3
https://doc.qt.io/qt-6/qml-qtquick-layouts-stacklayout.html#currentIndex-prop
Since 6.5, inserting/removing a new Item at an index less than or equal to the current index will increment/decrement the current index, but keep the current Item.