Fix two-way bindings

[lorentey]

As noted in issue #5, two-way bindings have not yet been updated to support transactional updates.

https://github.com/lorentey/GlueKit/blob/a0ed7f17fe0c05380270554c4ef79151f4cb473e/Sources/UpdatableValue.swift#L151-L171

Update the code to take transactions into account. This is more difficult than it seems! E.g., when b is bound to a, a beginTransaction update from a should immediately trigger the same from b. However, b's resulting beginTransaction message must not loop back to a, even though there is a two-way binding between them!

[lorentey]

Such two-way bindings may not be the correct abstraction; these tweets by @pyrtsa suggest alternative ways to approach the underlying problem.
https://twitter.com/pyrtsa/status/790167771900411905
https://twitter.com/pyrtsa/status/790173012075155457

[lorentey]

This is now mostly fixed. However, trying to create a two-way binding (or trying to close one down) during an active transaction isn't supported—that would require a nontrivial change to our transaction model.