Reimplement Exit App function

Now ties into the Jellyfin Web project's existing code for exiting.

native/nativeshell.js

@@ -8,7 +8,7 @@ const features = [
     "externallinks",
     "clientsettings",
     "multiserver",
-    "quitapp",
+    "exitmenu",
     "remotecontrol",
     "fullscreenchange",
     "filedownload",
@@ -56,10 +56,6 @@ window.NativeShell = {
         showSettingsModal();
     },
 
-    quitApp() {
-        window.api.system.exit();
-    },
-
     getPlugins() {
         return plugins;
     }
@@ -137,6 +133,9 @@ window.NativeShell.AppHost = {
     },
     deviceName() {
         return viewdata.deviceName;
+    },
+    exit() {
+        window.api.system.exit();
     }
 };