GSD lets us filter based on two things at present: Signal
name and sender.  Sender is rarely used properly in Hiss
(I'm not even sure what proper usage is) and we just use it
two get a second item to filter on.

Thus, it would be great to be able to filter based on any
keyword:
self.app.connect(function, signal, kw1=val1, kw2=val2, ...)

I imagine the implementation shouldn't be too difficult to
seemlessly overlay onto the existing Dispatcher.