* src/calls-manager.h: Add lookup function to header
* src/calls-manager.c: Implement lookup function to be used for the call
history, call notifier and call display
* src/calls-call-holder.c: Use calls_manager_get_contact_name ()
* src/calls-notifier.c: Use calls_manager_get_contact_name ()
The manager is a central place for handling data e.g. calls history and
provider managment.
This introduces only the base for feature work. Therefore it manages
only the provider for now, but the manager isn't yet used anywhere.
The propagets events from the provider and origins. It also adds a new
signal called `error`. The `error` signal should be emited only when
something went wrong and we need to inform the user about it, containing
the message to be displayed to the user. Windows should connect to the
event and display a in-app-notification on error. This event isn't
emitted, because the plugins don't give us a usable error.
