Add chains of observables

[felixb-wix]

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[felixb-wix]

Concept:

• both kinds of observables have the same interface ObservableState
• the chained observable takes multiple observables as an input ("sources"), and subscribes to each. When any of the source observables raises a notification, the chained observables's own subscribers get notified.

What I do in this PR:

• remove current() from ObservableState
• add createChainedObservable() to Shell

Open issues:

• the chained observables don't have any lifecycle mechanism (like React's componentWillUnmount), so currently they have no way to unsubscribe from the sources. A possible solution is use WeakRef for the subscribers.
• it is possible to accidentally create endless loops