window: extract XWindow primitive; XNotifications stops carrying focus

The window-focus signal had no business living on the notifications
primitive — it was there because the same NotificationsPlugin happened
to observe NSApplication active/resign for its own reasons. Splitting
it into a sibling XWindow primitive (with its own WindowPlugin on
macOS, ux/window/events) lets future consumers — paused video,
deferred-work scheduling, dock badge counts — read focus state without
pulling in UNUserNotificationCenter.

XNotifications now only exposes notification I/O (show/cancel + tap +
authorization). The 'type:focus' event-channel branch is gone.

macos/Classes/XPlugin.swift

@@ -13,6 +13,7 @@ public class XPlugin: NSObject, FlutterPlugin {
             UxVideoPlayerPlugin(),
             UrlPlugin(),
             NotificationsPlugin(),
+            WindowPlugin(),
         ]
         for plugin in plugins {
             plugin.register(with: registrar)