Make user choose effect of saving inherited properties

Using the IDE to view inherited values works great, but the effect of editing one is not obvious.

At the moment, if you modify the displayed value and then save it, the editor creates an overriding property on the object being edited. This is perfectly reasonable behaviour (though note that, per #257, the editor gives bad feedback about what has just happened at the moment).

It might alternatively be useful or even expected that editing the inherited property actually modifies the value on the (appropriate) prototype object. To that end:

* There should be some way for the user to choose which object the value will be saved on, in the range of (prototype object from which it is currently inherited to this object).
* Maybe the user should be forced to decide (or at least make an initial decision) before they are actually able to edit the value.