NotificationServer: Add a system service for desktop notifications

This patch adds NotificationServer, which runs as the "notify" user
and provides an IPC API for desktop notifications.

LibGUI gains the GUI::Notification class for showing notifications.

NotificationServer is spawned on demand and will unspawn after
dimissing all visible notifications. :^)

Finally, this also comes with a small /bin/notify utility.

Libraries/LibGUI/Notification.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <LibCore/Object.h>
+
+namespace GUI {
+
+class Notification : public Core::Object {
+    C_OBJECT(Notification);
+
+public:
+    virtual ~Notification() override;
+
+    const String& text() const { return m_text; }
+    void set_text(const String& text) { m_text = text; }
+
+    const String& title() const { return m_title; }
+    void set_title(const String& title) { m_title = title; }
+
+    void show();
+
+private:
+    Notification();
+
+    String m_title;
+    String m_text;
+};
+
+}