Allow attached types to execute code at component completion

In https://codereview.qt-project.org/c/qt/qtdeclarative/+/554020/25/src/quicktemplates/qquickcontextmenu.cpp#121 I was trying to defer a property of an attached type. It doesn't work because componentComplete is not called for types created from C++. Fabian said:

Tor Arne actually also had a use case for hooking into the QQmlFinalizerHook logic from an attached property; but we avoided that so far. I'd like to avoid adding that API, but if you really have that use case, we can add it