Actor: Added a new and simpler Action/Actor mechanism.

This allows exposing Actions to the UI when they are not specifically
part of the generic API.

An object able to execute Actions is an Actor. An Actor is exposing its
own Actions which can also be pulled from an ActionProvider (e.g. the
CallCore could provide call Actions to a Contact).
GActorMenu: Added a GActorMenu object.

This object is able to expose Actor Actions to the UI. It also acts as a
bridge between Actors and GIO actions.
Use the new Actor subsystem. (still incomplete).
Updated TODO.