Move panel into an old-panel subdirectory

We expect to replace this with applets running under
https://github.com/pop-os/cosmic-dock-epoch.

src/main.rs

@@ -1,25 +0,0 @@
-use gtk4::{glib, prelude::*};
-
-mod application;
-mod dbus_service;
-mod deref_cell;
-mod mpris;
-mod mpris_player;
-mod notification_list;
-mod notification_popover;
-mod notification_widget;
-mod notifications;
-mod popover_container;
-mod status_area;
-mod status_menu;
-mod status_notifier_watcher;
-mod time_button;
-mod window;
-
-use application::PanelApp;
-
-fn main() {
-    glib::MainContext::default()
-        .with_thread_default(|| PanelApp::new().run())
-        .unwrap();
-}