config: allow disabling the built-in bar

src/compositor.rs

@@ -351,6 +351,7 @@ fn start_compositor2(
         head_names: Default::default(),
         head_managers: Default::default(),
         head_managers_async: Default::default(),
+        show_bar: Cell::new(true),
     });
     state.tracker.register(ClientId::from_raw(0));
     create_dummy_output(&state);