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-manager.h

@@ -42,6 +42,7 @@ typedef enum
   CALLS_MANAGER_STATE_NO_PLUGIN,
   CALLS_MANAGER_STATE_NO_PROVIDER,
   CALLS_MANAGER_STATE_NO_ORIGIN,
+  CALLS_MANAGER_STATE_NO_VOICE_MODEM,
   CALLS_MANAGER_STATE_READY,
 } CallsManagerState;