Ladybird: Allow posting events to the Qt event loop from other threads

Previously, a QTimer was used to start processing of our event queue in
the main Qt event loop. Unfortunately, QTimers are not thread-safe, and
disallow starting of a timer from a different thread than it was
created in.

Instead, use a dummy QObject to post a custom QEvent to the main loop
from whatever thread we like, and process our event queue when it is
received by our dummy object.

Ladybird/EventLoopImplementationQtEventTarget.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2023, Gregory Bertilson <zaggy1024@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <QEvent>
+
+#include "EventLoopImplementationQt.h"
+
+namespace Ladybird {
+
+class EventLoopImplementationQtEventTarget final : public QObject {
+    Q_OBJECT
+
+public:
+    virtual bool event(QEvent* event) override;
+};
+
+}