shared_script_properties_observe: only run callback when observed value changes

[CogentRedTester]

Even though these functions aren't documented a number of scripts have started to write to this property. This commit adds the bare minimum optimisation to the shared_script_property_observe function so that the callback is only run when the observed sub-property actually changes.

[avih]

I don't think we should take this. If they care, users should take into account that it can be called even without change, just like they need to take that into account while observing any other property.

[Dudemanguy]

Since we have user-data now, I think this is obsolete. The fix is to just use that instead so I'll close this if you don't mind.