Implement an editor to customize the touchscreen controls (#14933)

- The editor is accessible via the pause menu and the settings menu.
- Buttons can be moved via drag & drop.
- Buttons can be added/removed. The grid menu added by #14918 is used to show
  all buttons not included in the layout.
- Custom layouts are responsive and adapt to changed screen size / DPI /
  hud_scaling.
- The layout is saved as JSON in the "touch_layout" setting.

src/irr_gui_ptr.h

@@ -0,0 +1,28 @@
+// Luanti
+// SPDX-License-Identifier: LGPL-2.1-or-later
+// Copyright (C) 2024 grorp, Gregor Parzefall <grorp@posteo.de>
+
+#pragma once
+#include <memory>
+#include "IGUIElement.h"
+
+// We cannot use irr_ptr for Irrlicht GUI elements we own.
+// Option 1: Pass IGUIElement* returned by IGUIEnvironment::add* into irr_ptr
+//           constructor.
+//           -> We steal the reference owned by IGUIEnvironment and drop it later,
+//           causing the IGUIElement to be deleted while IGUIEnvironment still
+//           references it.
+// Option 2: Pass IGUIElement* returned by IGUIEnvironment::add* into irr_ptr::grab.
+//           -> We add another reference and drop it later, but since IGUIEnvironment
+//           still references the IGUIElement, it is never deleted.
+// To make IGUIEnvironment drop its reference to the IGUIElement, we have to call
+// IGUIElement::remove, so that's what we'll do.
+template <typename T>
+std::shared_ptr<T> grab_gui_element(T *element)
+{
+	static_assert(std::is_base_of_v<irr::gui::IGUIElement, T>,
+			"grab_gui_element only works for IGUIElement");
+	return std::shared_ptr<T>(element, [](T *e) {
+		e->remove();
+	});
+}