LibCore: Add a forward declaration header

This patch adds <LibCore/Forward.h> and uses it in various places to
shrink the header dependency graph.

Libraries/LibCore/Forward.h

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+namespace Core {
+
+class ArgsParser;
+class ChildEvent;
+class ConfigFile;
+class CustomEvent;
+class DateTime;
+class DirIterator;
+class ElapsedTime;
+class Event;
+class EventLoop;
+class File;
+class HttpJob;
+class HttpRequest;
+class HttpResponse;
+class IODevice;
+class LocalServer;
+class LocalSocket;
+class MimeData;
+class NetworkJob;
+class NetworkResponse;
+class Notifier;
+class Object;
+class ProcessStatisticsReader;
+class Socket;
+class SocketAddress;
+class TCPServer;
+class TCPSocket;
+class TimerEvent;
+class UdpServer;
+class UdpSocket;
+
+enum class TimerShouldFireWhenNotVisible;
+
+}