manager: Differentiate the NO_ORIGIN state

This avoids confusion when only the SIP plugin is loaded, but no account
is being used.

For this we introduce two new functions in the abstract CallsProvider class:
`is_modem()` to indicate whether the the provider deals with PTSN telephony and
`is_operational()` which by default simply checks if there any avalaible origins

src/calls-main-window.c

@@ -315,10 +315,14 @@ state_changed_cb (CallsMainWindow *self,
     case CALLS_MANAGER_STATE_READY:
       break;
 
-    case CALLS_MANAGER_STATE_NO_ORIGIN:
+    case CALLS_MANAGER_STATE_NO_VOICE_MODEM:
       error = _("Can't place calls: No voice-capable modem available");
       break;
 
+    case CALLS_MANAGER_STATE_NO_ORIGIN:
+      error = _("Can't place calls: No modem or VoIP account available");
+      break;
+
     case CALLS_MANAGER_STATE_UNKNOWN:
     case CALLS_MANAGER_STATE_NO_PROVIDER:
       error = _("Can't place calls: No backend service");