ENH: Added ability to mark items defined in a view as read only.
Syntax is:
<View Type="Instanced" Title="Global Constants">
<InstancedAttributes>
<Att Name="physics" Type="physics" />
<Att Name="physical-constants" Type="physical-constants">
<ItemViews>
<View Item="absolute-zero" Type="Default" ReadOnly="true"/>
</ItemViews>
</Att>
</InstancedAttributes>
</View>