When we deprecate a property, we can

• QT_DEPRECATED_SINCE the setter and getter (and bindable, if any) to get warnings at compile-time when using them
• QT_MOC_COMPAT the NOTIFY signal (and the setter-as-a-slot?) to get a warning at run-time if an old-style connect is done involving the signal (and slot?)
  • though this lacks version information

and that already goes a long way.

But we have no way to mark the property itself as deprecated to issue a runtime warning when the property is accesses through QObject::setProperty()/QObject::property().

Acceptance criteria:

• provide the necessary tools to comprehensively deprecate Q_PROPERTYs
  • decide whether version information is essential and if so, add support
• document how to use these tools in https://wiki.qt.io/Deprecation and/or as a QUIP