Allow RX library authors to group observables as native operators

[niik]

I would like to make it possible for libraries like ReactiveUI to signal that a series of observables should be grouped together. The WhenAny method in ReactiveUI for example should be considered an operator and should show up as a single node instead of the graph of observables stringed together to create it.

My current plan for this is to use the System.ComponentModel.CategoryAttribute so that libraries don't have to take a dependency on RxSpy. WhenAny would then be decorated with [Category("RxSpy.Operator")] or something to that effect.