call: Add API to query whether this call can do DTMF

This allows enable or disable UI elements.

src/calls-call.c

@@ -358,6 +358,19 @@ tone_key_is_valid (gchar key)
     ||  key == '#';
 }
 
+/**
+ * calls_call_can_dtmf:
+ * @self: a #CallsCall
+ *
+ * Returns: %TRUE if this call supports DTMF, %FALSE otherwise
+ */
+gboolean
+calls_call_can_dtmf (CallsCall *self)
+{
+  g_return_val_if_fail (CALLS_IS_CALL (self), FALSE);
+
+  return CALLS_CALL_GET_CLASS (self)->tone_start != calls_call_real_tone_start;
+}
 
 /**
  * calls_call_tone_start:

src/calls-call.h

@@ -70,6 +70,7 @@ gboolean         calls_call_get_inbound    (CallsCall *self);
 const char      *calls_call_get_protocol   (CallsCall *self);
 void             calls_call_answer         (CallsCall *self);
 void             calls_call_hang_up        (CallsCall *self);
+gboolean         calls_call_can_dtmf       (CallsCall *self);
 void             calls_call_tone_start     (CallsCall *self,
                                             gchar      key);
 gboolean         calls_call_tone_stoppable (CallsCall *self);