Add GtkDial:can_control property

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

src/window-levels.c

@@ -103,6 +103,7 @@ GtkWidget *create_levels_controls(struct alsa_card *card) {
       // create the meter widget and attach to the grid
       GtkWidget *meter = gtk_dial_new_with_range(-80, 0, 0, 0);
       gtk_dial_set_taper(GTK_DIAL(meter), GTK_DIAL_TAPER_LINEAR);
+      gtk_dial_set_can_control(GTK_DIAL(meter), FALSE);
       gtk_widget_set_sensitive(meter, FALSE);
       card->meters[meter_num++] = meter;
       gtk_grid_attach(GTK_GRID(grid), meter, j + 1, i + 1, 1, 1);