Notification: Allow getting window by ID

This is a neccessity so a connection can get the notification it is
connected to for later updates.

Userland/Services/NotificationServer/NotificationWindow.cpp

@@ -128,4 +128,10 @@ NotificationWindow::~NotificationWindow()
 {
 }
 
+RefPtr<NotificationWindow> NotificationWindow::get_window_by_id(i32 id)
+{
+    auto window = s_windows.get(id);
+    return window.value_or(nullptr);
+}
+
 }