FileSystemAccessServer+TextEditor: Implement cross-process modal prompts

This transitions from synchronous IPC calls to asynchronous IPC calls
provided through a synchronous interface in LibFileSystemAccessClient
which allows the parent Application to stay responsive.

It achieves this with Promise which is pumping the Application event
loop while waiting for the Dialog to respond with the user's action.

LibFileSystemAccessClient provides a lazy singleton which also ensures
that FileSystemAccessServer is running in the event of a crash.

This also transitions TextEditor into using LibFileSystemAccessClient.

Userland/Libraries/LibFileSystemAccessClient/Client.h

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2021, timmot <tiwwot@protonmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <FileSystemAccessServer/FileSystemAccessClientEndpoint.h>
+#include <FileSystemAccessServer/FileSystemAccessServerEndpoint.h>
+#include <LibCore/File.h>
+#include <LibCore/Promise.h>
+#include <LibIPC/ServerConnection.h>
+
+namespace FileSystemAccessClient {
+
+struct Result {
+    i32 error;
+    Optional<i32> fd;
+    Optional<String> chosen_file;
+};
+
+class Client final
+    : public IPC::ServerConnection<FileSystemAccessClientEndpoint, FileSystemAccessServerEndpoint>
+    , public FileSystemAccessClientEndpoint {
+    C_OBJECT(Client)
+
+public:
+    Result request_file(i32 parent_window_id, String const& path, Core::OpenMode mode);
+    Result open_file(i32 parent_window_id);
+    Result save_file(i32 parent_window_id, String const& name, String const ext);
+
+    static Client& the();
+
+protected:
+    void die() override;
+
+private:
+    explicit Client()
+        : IPC::ServerConnection<FileSystemAccessClientEndpoint, FileSystemAccessServerEndpoint>(*this, "/tmp/portal/filesystemaccess")
+    {
+    }
+
+    virtual void handle_prompt_end(i32 error, Optional<IPC::File> const& fd, Optional<String> const& chosen_file) override;
+
+    RefPtr<Core::Promise<Result>> m_promise;
+};
+
+}