Improve/create appimage 2026 08 04

[pedrolcl]

• Validate required args and paths, use portable arch check
• Allow passing APPIMAGETOOL_PATH / APPIMAGETOOL_VERSION
• Safer quoting of paths, better error messages, and capture appimagetool output on failure
• Place output AppImage in WORKDIR and prefer relative symlinks inside AppDir
• Add usage docs and example in the file header

[pedrolcl]

Problems:

• There is no license, it should be on line 2: "# SPDX-License-Identifier: BSD-3-Clause".
• There is no indication of the minimum cmake version. Should have: "cmake_minimum_required(VERSION 3.16)".
• There is no validation of unparsed arguments. Should have something like:

    if(ARGS_UNPARSED_ARGUMENTS)
        message(FATAL_ERROR ""CREATE_APPIMAGE: unparsed arguments ${ARGS_UNPARSED_ARGUMENTS}")
    endif()

• Comment about using the CPack AppImage generator in CMake >= 4.2 instead of this function.
• Unrelated to this function. The AppImage does not include both xcb and wayland platform plugins:
  in main CMakeLists.txt: qt_generate_deploy_app_script() should have a INCLUDE_PLUGINS argument (Linux).