LibCore: Big first step towards pluggable Core::EventLoop

The EventLoop is now a wrapper around an EventLoopImplementation.
Our old EventLoop code has moved into EventLoopImplementationUnix and
continues to work as before.

The main difference is that all the separate thread_local variables have
been collected into a file-local ThreadData data structure.

The goal here is to allow running Core::EventLoop with a totally
different backend, such as Qt for Ladybird.

Userland/Libraries/LibCore/EventLoopImplementation.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2022-2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+#include <LibCore/Forward.h>
+
+namespace Core {
+
+class ThreadEventQueue;
+
+class EventLoopImplementation {
+public:
+    virtual ~EventLoopImplementation();
+
+    enum class PumpMode {
+        WaitForEvents,
+        DontWaitForEvents,
+    };
+
+    void post_event(Object& receiver, NonnullOwnPtr<Event>&&);
+
+    virtual int exec() = 0;
+    virtual size_t pump(PumpMode) = 0;
+    virtual void quit(int) = 0;
+    virtual void wake() = 0;
+
+    virtual void deferred_invoke(Function<void()>) = 0;
+
+    virtual int register_timer(Object&, int milliseconds, bool should_reload, TimerShouldFireWhenNotVisible) = 0;
+    virtual bool unregister_timer(int timer_id) = 0;
+
+    virtual void register_notifier(Notifier&) = 0;
+    virtual void unregister_notifier(Notifier&) = 0;
+
+    // FIXME: These APIs only exist for obscure use-cases inside SerenityOS. Try to get rid of them.
+    virtual void unquit() = 0;
+    virtual bool was_exit_requested() const = 0;
+    virtual void notify_forked_and_in_child() = 0;
+    virtual int register_signal(int signal_number, Function<void(int)> handler) = 0;
+    virtual void unregister_signal(int handler_id) = 0;
+
+protected:
+    EventLoopImplementation();
+
+    ThreadEventQueue& m_thread_event_queue;
+};
+
+}