LiveObject: Allow empty populate_menu.

This is required due to the recent changes. We are migrating some
LiveObjects to Actors. This is required during the transition.
ContactActor: Call add_action method of parent in add_action.

This prevents code duplication between ContactActor and Actor objects.
ContactAction: Allow ContactAction to be validated with several testers.

Also added typedefs for better code clarity.
Actor: Add a list of actions names to remember insertion order.

This will allow ordering menus following insertion order instead of
relying on the keys orders of the ActionMap.

However, we keep the ActionMap for performance reasons.
LocalCluster: Migrated to Actor/ContactActor API.

This allows us getting rid of the Roster bridget.
Main Window: Got rid of the Trigger.

This is not required anymore. The Trigger was executing the most
significant action of a Trigger object.

You can simply use global actions exported by the object itself.