Add getter/setter type to MessageView

[Matthias Schoettle]

Instead of having a completely defined MessageView for getters and setters, an enumeration type should be added to the MessageView denoting whether it is normal, getter or setter.

A getter/setter may not have a specification (just like message views for partial operations). The constraint should be adjusted accordingly.

The getter and setter need to somehow reference the property that it corresponds to. The weaver can, when advicing a getter/setter create the message view structure. The generator can produce the correct code for a getter/setter message view.

[Matthias Schoettle]

In order to advice a message view or define some additional behaviour (but still keeping it a getter/setter), we need an actual message view.

Updated and moved to #367.