Escalator: Major rework and enable forwarding of stdin/stdout

- Escalator now uses `posix_spawn` instead of `exec`
- Escalator is now able to forward stdin and stdout
- A few general changes to improve code quality

Userland/Applications/Escalator/EscalatorWindow.h

@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2022, Ashley N. <dev-serenity@ne0ndrag0n.com>
  * Copyright (c) 2022, the SerenityOS developers.
+ * Copyright (c) 2023, Fabian Dellwing <fabian@dellwing.net>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -23,6 +24,8 @@ public:
         StringView description;
         Core::Account current_user;
         bool preserve_env { false };
+        bool forward_stdin { false };
+        bool forward_stdout { false };
     };
 
     virtual ~EscalatorWindow() override = default;
@@ -38,6 +41,8 @@ private:
     StringView m_executable;
     Core::Account m_current_user;
     bool m_preserve_env { false };
+    bool m_forward_stdin { false };
+    bool m_forward_stdout { false };
 
     RefPtr<GUI::ImageWidget> m_icon_image_widget;
     RefPtr<GUI::Button> m_ok_button;