Add a connection that weak references its listener
Some tidying.
Make weak references stuff not cause GWT to choke.

Also added note about how weak references are not actually weak in JavaScript.
Someday JavaScript may support weak references in the standard (Chrome has them
as an experimental feature, I think), and we'll make this work properly then.
Add weak connections to Signal as well.

We'll see how well these work out on Signal and Value, and then roll them out
to RList, RMap and RSet if the experiment turns out to be a success.