More snippets for QQuickAttachedPropertyPropagator

https://doc.qt.io/qt-6/qquickattachedpropertypropagator.html#details instructs developers to "Define set/inherit/propagate/reset functions for each property as needed".

Out of the 4 functions that must be defined for a property, the page only shows code snippets for 2 of them: "propagateTheme()" and "resetTheme()"

Suggestion
Also add snippets to https://doc.qt.io/qt-6/qquickattachedpropertypropagator.html to demonstrate implementations of "setTheme()" and "inheritTheme()". Such implementations are available at https://code.qt.io/cgit/qt/qtdeclarative.git/tree/examples/quickcontrols/attachedstyleproperties/MyStyle/mystyle.cpp?h=6.8