Add support for independent set/get visibility/access of fields/properties in class diagrams

[mwhincup]

Matlab allows you to independently define the get access (visibility) and the set access of a property.
Using the example from the plantuml documentation, one could specify, e.g.
public get access and protected set access for field1
and
protected get access and private set access for field2
thus:

@startuml

class Dummy {
 +#field1
 #-field2
 ~method1()
 +method2()
}

@enduml

[arnaudroques]

I'm not sure to understand the request, here.