Main Window: Use global action name instead of the local one.
Actor: Added removed/added signals + method to remove an action.

This allows actions to be dynamically added and removed.
ActorMenu: Added support for added/removed Actor signals.

This allows ContactActorMenu objects to be modified dynamically.
CallCore: Added dynamic call/transfer actions.

The call action is disabled as soon as we are in a call (we only support
one call at a time).
The transfer action is added as soon as we are in a call (all our call
managers do support, and must support transfer).
Main Window: Fixed call button. It should not be a toggle.
Main Window: Fixed contact menu being endlessly added.