Add support for dynamic data binding

Pivot currently supports data binding via a load/store model that maps well to client/server applications such as REST clients. However, the dynamic data binding supported by other platforms (where a property of a target element is automatically updated whenever a source value changes) is also useful in certain circumstances.

Given recent updates to BXMLSerializer, such dynamic binding would be relatively straightforward to implement. This syntax would be a cue to the serializer to register a change listener on the given variable and propagate any changes to the bound property:

<Label text="${myTextProperty}"/>

It should also be possible to create (and remove) such bindings programmatically in code.