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/gui/touchscreeneditor.h

@@ -0,0 +1,81 @@
+// Luanti
+// SPDX-License-Identifier: LGPL-2.1-or-later
+// Copyright (C) 2024 grorp, Gregor Parzefall <grorp@posteo.de>
+
+#pragma once
+
+#include "touchscreenlayout.h"
+#include "modalMenu.h"
+
+#include <memory>
+#include <unordered_map>
+
+class ISimpleTextureSource;
+namespace irr::gui
+{
+	class IGUIImage;
+}
+
+class GUITouchscreenLayout : public GUIModalMenu
+{
+public:
+	GUITouchscreenLayout(gui::IGUIEnvironment* env,
+            gui::IGUIElement* parent, s32 id,
+            IMenuManager *menumgr, ISimpleTextureSource *tsrc);
+	~GUITouchscreenLayout();
+
+	void regenerateGui(v2u32 screensize);
+	void drawMenu();
+	bool OnEvent(const SEvent& event);
+
+protected:
+	std::wstring getLabelByID(s32 id) { return L""; }
+	std::string getNameByID(s32 id) { return ""; }
+
+private:
+	ISimpleTextureSource *m_tsrc;
+
+	ButtonLayout m_layout;
+	v2u32 m_last_screensize;
+	s32 m_button_size;
+
+	enum class Mode {
+		Default,
+		Dragging,
+		Add,
+	};
+	Mode m_mode = Mode::Default;
+
+	std::unordered_map<touch_gui_button_id, std::shared_ptr<gui::IGUIImage>> m_gui_images;
+	// unused if m_mode == Mode::Add
+	std::unordered_map<touch_gui_button_id, v2s32> m_gui_images_target_pos;
+	void clearGUIImages();
+	void regenerateGUIImagesRegular(v2u32 screensize);
+	void regenerateGUIImagesAddMode(v2u32 screensize);
+	void interpolateGUIImages();
+
+	// interaction state
+	bool m_mouse_down = false;
+	v2s32 m_last_mouse_pos;
+	touch_gui_button_id m_selected_btn = touch_gui_button_id_END;
+
+	// dragging
+	ButtonLayout m_last_good_layout;
+	std::vector<core::recti> m_error_rects;
+	void updateDragState(v2u32 screensize, v2s32 mouse_movement);
+
+	// add mode
+	ButtonLayout m_add_layout;
+	std::vector<std::shared_ptr<gui::IGUIStaticText>> m_add_button_titles;
+
+	// Menu GUI elements
+	std::shared_ptr<gui::IGUIStaticText> m_gui_help_text;
+
+	std::shared_ptr<gui::IGUIButton> m_gui_add_btn;
+	std::shared_ptr<gui::IGUIButton> m_gui_reset_btn;
+	std::shared_ptr<gui::IGUIButton> m_gui_done_btn;
+
+	std::shared_ptr<gui::IGUIButton> m_gui_remove_btn;
+
+	void regenerateMenu(v2u32 screensize);
+};