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.

doc/menu_lua_api.md

@@ -223,6 +223,7 @@ GUI
 * `core.set_clouds(<true/false>)`
 * `core.set_topleft_text(text)`
 * `core.show_keys_menu()`
+* `core.show_touchscreen_layout()`
 * `core.show_path_select_dialog(formname, caption, is_file_select)`
   * shows a path select dialog
   * `formname` is base name of dialog response returned in fields