Add two way binding for check button state

[glopesdev]

This PR introduces two way binding between view and view-model state for check-box button style controls in the package, i.e. in this case the Checked property. In general the same pattern will be followed for all other controls with state.

Almost all primitive controls are concerned with managing a single piece of state, and we want any changes in this state to be propagated as an observable sequence. To allow two way binding we replaced the UI change event source with a BehaviorSubject that is responsible for storing the state. Apart from signaling all changes, this subject can also be used as a binding source by the UI control, so that any external changes to the state are automatically reflected in the UI. Finally, the state is exposed through a configurable property which can be externalized in the workflow.

Aside from two way binding, this allows for state to persist between opening and closing of visualizer windows, and also for dynamically changing the state using other workflow inputs by externalizing and mapping the state property.