EkigaApp: Make sure there is only one call window at any time.
Call: A call is now an Actor and a LiveObject.

- An Actor can register Actions (hold, hangup, transfer, ...)
  dynamically, depending on the exact implementation of the call.
- A LiveObject can interact with users through Forms. (e.g. to request
  the user to enter a URI where to transfer a call).
CallCore: Add a new signal emitted when a call is created.

This signal is emitted before the call setup signal.