Add form property value access API

Previously forms and other editors would be continiously connected to the datasource item and would directly write the changed properties. Due to that actions that operate over such items could fetch the changed values directly from them.

Now forms only write the changes to an item when some sort of save action is fired. All the pending changes are effectively stored in the form's binder/field components. API that would bridge to the bindings is required.

At the moment we have:

    <V> Stream<V> getFieldValue(Class<V> valueType);

This would fetch all the values of the same type, which has obvious flaws.

As a replacement I suggest that we add a method that let's access the property value (optionally typed) by property name and, since we support i18n, by locale (optionally as well).