call: Move state property into base class

This let's us get rid of a lot of duplication in the derived classes.

Additionally we set the initial state to CALLS_CALL_STATE_INCOMING if
inbound is TRUE and CALLS_CALL_STATE_DIALING otherwise.

plugins/sip/calls-sip-origin.c

@@ -575,7 +575,7 @@ sip_i_state (int              status,
     return;
   }
 
-  calls_sip_call_set_state (call, state);
+  calls_call_set_state (CALLS_CALL (call), state);
 }