Remove user-control of level meters

jandejongh reported:

In the USB Levels display, user can (attempt to) set the level of the
"slider".
Expected: These "sliders" are read-only and should not allow setting a
value (makes no sense to me?).

Fixes: #78.

src/window-levels.c

@@ -104,6 +104,7 @@ 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, 1);
+      gtk_widget_set_sensitive(meter, FALSE);
       card->meters[meter_num++] = meter;
       gtk_grid_attach(GTK_GRID(grid), meter, j + 1, i + 1, 1, 1);
     }