before the disable property was a bit weird. Setting something to
disabled=true will disable all children of the widget that is changed.
However, only the update function of the children will get the false flag,
not the flag itself. Which means, to query the real disabled state, we
need to walk up the parent relations.
With this patch, every change to disabled will go through the disabled
property, which mean, a implementor can just overwrite the disabled
property, and adjust its internal state there. Just be carefull, a set
to disabled=true still might result in disabled=false. This makes the
function on_disable_update unneccesary. Which also cleans up the
Depends on D8016