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.cpp

@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) 2021, timmot <tiwwot@protonmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+// FIXME: LibIPC Decoder and Encoder are sensitive to include order here
+// clang-format off
+#include <LibGUI/WindowServerConnection.h>
+// clang-format on
+#include <LibCore/StandardPaths.h>
+#include <LibFileSystemAccessClient/Client.h>
+#include <LibGUI/Window.h>
+
+namespace FileSystemAccessClient {
+
+static RefPtr<Client> s_the = nullptr;
+
+Client& Client::the()
+{
+    if (!s_the || !s_the->is_open())
+        s_the = Client::construct();
+    return *s_the;
+}
+
+Result Client::request_file(i32 parent_window_id, String const& path, Core::OpenMode mode)
+{
+    m_promise = Core::Promise<Result>::construct();
+    auto window_server_client_id = GUI::WindowServerConnection::the().expose_client_id();
+
+    async_request_file(window_server_client_id, parent_window_id, path, mode);
+
+    return m_promise->await();
+}
+
+Result Client::open_file(i32 parent_window_id)
+{
+    m_promise = Core::Promise<Result>::construct();
+    auto window_server_client_id = GUI::WindowServerConnection::the().expose_client_id();
+
+    async_prompt_open_file(window_server_client_id, parent_window_id, Core::StandardPaths::home_directory(), Core::OpenMode::ReadOnly);
+
+    return m_promise->await();
+}
+
+Result Client::save_file(i32 parent_window_id, String const& name, String const ext)
+{
+    m_promise = Core::Promise<Result>::construct();
+    auto window_server_client_id = GUI::WindowServerConnection::the().expose_client_id();
+
+    async_prompt_save_file(window_server_client_id, parent_window_id, name.is_null() ? "Untitled" : name, ext.is_null() ? "txt" : ext, Core::StandardPaths::home_directory(), Core::OpenMode::Truncate | Core::OpenMode::WriteOnly);
+
+    return m_promise->await();
+}
+
+void Client::handle_prompt_end(i32 error, Optional<IPC::File> const& fd, Optional<String> const& chosen_file)
+{
+    VERIFY(m_promise);
+
+    if (error == 0) {
+        m_promise->resolve({ error, fd->take_fd(), *chosen_file });
+    } else {
+        m_promise->resolve({ error, {}, chosen_file });
+    }
+}
+
+void Client::die()
+{
+    if (m_promise)
+        handle_prompt_end(ECONNRESET, {}, "");
+}
+
+}