Implement #165 - enable/disable fieldsets

#165 implemented as follows:
* Configure if disabling is allowed per Archetype
* Add "power off" icon to fieldsets that can be disabled
* Dim the icon and label of a disabled fieldset
* Nested Archetypes can have fieldsets enabled/disabled (if allowed) independantly of their parent fieldset
* ```ArchetypeModel.GetEnumerator()``` only returns fieldsets that aren't disabled - default value for ```ArchetypeFieldsetModel.Disabled``` is false, so this should be backwards compatible

Screenshot 1
![165-01](https://cloud.githubusercontent.com/assets/7405322/3420582/975e6c20-feb2-11e3-8ecd-fed4ff9a9b20.png)
Screenshot 2
![165-02](https://cloud.githubusercontent.com/assets/7405322/3420583/991e64de-feb2-11e3-8353-ad6458e44ff7.png)