LibWebView+LibCore: Manage process lifecycle using a SIGCHLD handler

This large commit also refactors LibWebView's process handling to use
a top-level Application class that uses a new WebView::Process class to
encapsulate the IPC-centric nature of each helper process.

Userland/Libraries/LibWebView/Process.h

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2024, Andrew Kaster <akaster@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <AK/WeakPtr.h>
+#include <LibCore/Process.h>
+#include <LibIPC/Connection.h>
+#include <LibWebView/ProcessType.h>
+
+namespace WebView {
+
+class Process {
+    AK_MAKE_NONCOPYABLE(Process);
+    AK_MAKE_DEFAULT_MOVABLE(Process);
+
+public:
+    Process(ProcessType type, RefPtr<IPC::ConnectionBase> connection, Core::Process process);
+    ~Process();
+
+    ProcessType type() const { return m_type; }
+    Optional<String> const& title() const { return m_title; }
+    void set_title(Optional<String> title) { m_title = move(title); }
+
+    template<typename ConnectionFromClient>
+    Optional<ConnectionFromClient&> client()
+    {
+        if (auto strong_connection = m_connection.strong_ref())
+            return verify_cast<ConnectionFromClient>(*strong_connection);
+        return {};
+    }
+
+    pid_t pid() const { return m_process.pid(); }
+
+private:
+    Core::Process m_process;
+    ProcessType m_type;
+    Optional<String> m_title;
+    WeakPtr<IPC::ConnectionBase> m_connection;
+};
+
+}