* Add function to initialize git submodules

* Use function for retrieving git submodules

* Add GPL header

There are files now in Mudlet that seem to have been forgotten from being
added to these variables so they can be seen in Qt Creator, they are:
* OTHER_FILES:
  * ./.devcontainer/Dockerfile (added by Mudlet#4087)
  * ./.devcontainer/devcontainer.json (added by Mudlet#4087)
  * ./.devcontainer/library-scripts/desktop-lite-debian.sh (added by Mudlet#4088)
  * ./.github/workflows/generate-changelog.yml (added by Mudlet#6006)
  * ./CI/appveyor.validate_deployment.ps1 (added by Mudlet#3493)
  * ./CI/generate-changelog.lua (renamed from
      ./CI/generate-ptb-changelog.lua by Mudlet#6002, that file created by Mudlet#3517)
  * ./CI/travis.validate_deployment.sh (added by Mudlet#3493)
  * ./CI/update-autocompletion.lua (added by Mudlet#3452)
  * ./dangerfile.js (added by Mudlet#5489)
  * ./docker/.env.template (added by Mudlet#5047)
* DISTFILES:
  * ./.clang-tidy (added by Mudlet#4858)
  * ./cmake/FindLua51.cmake (added by Mudlet#3167)
  * ./cmake/FindSparkle.cmake (added Mudlet#3477)
  * ./cmake/FindZZIPLIB.cmake (added by Mudlet#3630)
  * ./cmake/IncludeOptionalModule.cmake (added by Mudlet#3174)
  * ./cmake/InitGitSubmodule.cmake (added by Mudlet#3171)

The following files have gone away:
* OTHER_FILES:
  * ./.github/workflows/whitespace-linter.yml (removed by Mudlet#4152)
  * ./.travis.yml (removed by Mudlet#5611)
  * ./README (renamed to ./dev_README by Mudlet#439, removed by Mudlet#1169)

Signed-off-by: Stephen Lyons <slysven@virginmedia.com>

…les (#6037)

This PR had three commits:
* The first commit just sorts in a case insensitive manner the files in the
OTHER_FILES and DISTFILES QMake variables. This is a precursor to making
changes to them.
* The second swaps some files between QMake OTHER_FILES and DISTFILES
variables. Technically the latter are for files that are needed to go into
a source tarball that the traditional GNU 'make dist' operation would
produce. As such it does not seem reasonable to include CI/CB control files
in it. Conversely the run-time LUA files that Mudlet needs definitely
should be included and so this commit moves them from the OTHER_FILES
variable, whose contents are merely files that we want to show up in the
Qt Creator IDE so they can be seen, searched and edited from there.
* The third changes the contents of QMake OTHER_FILES and DISTFILES
variables as there are files now in Mudlet that seem to have been forgotten
from being added to these variables so they can be seen in Qt Creator, they
are:
  * OTHER_FILES:
    * ./.devcontainer/Dockerfile (added by Setup Mudlet for C++ Github
                                                           Codespace #4087)
    * ./.devcontainer/devcontainer.json (added by Setup Mudlet for C++
                                                    Github Codespace #4087)
    * ./.devcontainer/library-scripts/desktop-lite-debian.sh (added by Open
                             Mudlet remotely using Github Codespaces #4088)
    * ./.github/workflows/generate-changelog.yml (added by Infrastructure:
                       add workflow to create changelog from the web #6006)
    * ./CI/appveyor.validate_deployment.ps1 (added by Add updater
                    validation to linux/macos/windows release builds #3493)
    * ./CI/generate-changelog.lua (renamed from
        ./CI/generate-ptb-changelog.lua by Infrastucture: update changelog
        generator to handle releases as well #6002, that file created by 
                          Add a changelog generator for PTB releases #3517)
    * ./CI/travis.validate_deployment.sh (added by Add updater validation
                               to linux/macos/windows release builds #3493)
    * ./CI/update-autocompletion.lua (added by Add action for automatic
                                  updates of Mudlet's autocompletion #3452)
    * ./dangerfile.js (added by Infrastructure: danger checks in JS #5489)
    * ./docker/.env.template (added by Docker development support #5047)
  * DISTFILES:
    * ./.clang-tidy (added by Add automated check to verify improvements
                                    against Mudlet's C++ style guide #4858)
    * ./cmake/FindLua51.cmake (added by Modernize cmake files (part 1)
                                                                     #3167)
    * ./cmake/FindSparkle.cmake (added Improve cmake osx #3477)
    * ./cmake/FindZZIPLIB.cmake (added by Build Mudlet for macOS with
                                                      Github Actions #3630)
    * ./cmake/IncludeOptionalModule.cmake (added by Cmake improve optional
                                                        module check #3174)
    * ./cmake/InitGitSubmodule.cmake (added by Cmake improve git submodule
                                                      initialization #3171)

Also, the following files have gone away:
  * OTHER_FILES:
    * ./.github/workflows/whitespace-linter.yml (removed by Delete
                                            whitespace-linter action #4152)
    * ./.travis.yml (removed by Infrastructure: delete .travis.yml #5611)
    * ./README (renamed to ./dev_README by Release 3.0 merge into master
                #439, removed by Merge 3.3.0 development into master #1169)

Signed-off-by: Stephen Lyons slysven@virginmedia.com