Fix modem addition/removal and deal with ModemManager appearing/vanishing

Modems being added or removed were not working.  To fix this, we pay
attention to the "object-removed" event and not just
"interface-removed".

Also, to deal with ModemManager appearing and vanishing, we add a
GDBus watch on ModemManager's D-Bus object.

Finally, we provide appropriate UI feedback when it's not possible to
make a call.

Closes #15
Closes #16

src/calls-provider.h

@@ -45,6 +45,7 @@ struct _CallsProviderInterface
 
 
 const gchar * calls_provider_get_name (CallsProvider *self);
+gchar * calls_provider_get_status (CallsProvider *self);
 GList * calls_provider_get_origins (CallsProvider *self);
 
 G_END_DECLS