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.cpp

@@ -0,0 +1,48 @@
+#include <LibGUI/Notification.h>
+#include <LibIPC/ServerConnection.h>
+#include <NotificationServer/NotificationClientEndpoint.h>
+#include <NotificationServer/NotificationServerEndpoint.h>
+
+namespace GUI {
+
+class NotificationServerConnection : public IPC::ServerConnection<NotificationClientEndpoint, NotificationServerEndpoint>
+    , public NotificationClientEndpoint {
+    C_OBJECT(NotificationServerConnection)
+public:
+    virtual void handshake() override
+    {
+        auto response = send_sync<Messages::NotificationServer::Greet>();
+        set_my_client_id(response->client_id());
+    }
+
+private:
+    NotificationServerConnection()
+        : IPC::ServerConnection<NotificationClientEndpoint, NotificationServerEndpoint>(*this, "/tmp/portal/notify")
+    {
+
+    }
+    virtual void handle(const Messages::NotificationClient::Dummy&) override {}
+};
+
+Notification::Notification()
+{
+}
+
+Notification::~Notification()
+{
+}
+
+static NotificationServerConnection& notification_server_connection()
+{
+    static NotificationServerConnection* connection;
+    if (!connection)
+        connection = &NotificationServerConnection::construct().leak_ref();
+    return *connection;
+}
+
+void Notification::show()
+{
+    notification_server_connection().post_message(Messages::NotificationServer::ShowNotification(m_text, m_title));
+}
+
+}