Add GtkDial:can_control property

If the dial can't be controlled, it shouldn't be dimmed when it's not
sensitive.

src/widget-gain.c

@@ -60,7 +60,8 @@ static void gain_updated(
 GtkWidget *make_gain_alsa_elem(
   struct alsa_elem *elem,
   int               zero_is_off,
-  int               widget_taper
+  int               widget_taper,
+  int               can_control
 ) {
   struct gain *data = g_malloc(sizeof(struct gain));
   data->elem = elem;
@@ -102,6 +103,8 @@ GtkWidget *make_gain_alsa_elem(
       2
     );
 
+  gtk_dial_set_can_control(GTK_DIAL(data->dial), can_control);
+
   data->label = gtk_label_new(NULL);
   gtk_widget_set_vexpand(data->dial, TRUE);