WIP: Fixed: Missing symlink python3 -> python3.8 in AppImages

Fixes #1306 (closed)

Reason for the error is that no python3 -> python3.8 symlink is in the AppImage package. Since Inkscape is searching for python3 and - as a fallback - for python, it selects python3 from system (/usr/local/bin/python3). This clashes with the AppImage environment. For details see explanation below.

See also #1050 (closed), inbox#2422 (closed), inbox#2656 (closed), partially inbox#1770 (moved)

@nathanal This is only half of the story: While Python extensions come up now, some lxml Python dependencies are not loaded, so in fact they fail now due incorrect inclusion of libs in generate.sh. I will fix this tomorrow, have no further time today. Hence, I mark this MR as WIP.

Detailed explanation

• Python interpreters Inkscape is looking for (script.cpp)

  const std::map<std::string, Script::interpreter_t> Script::interpreterTab = {
  #ifdef _WIN32
          { "perl",    {"perl-interpreter",    {"wperl"             }}},
          { "python",  {"python-interpreter",  {"pythonw"           }}},
  #elif defined __APPLE__
          { "perl",    {"perl-interpreter",    {"perl"              }}},
          { "python",  {"python-interpreter",  {"python3"           }}},
  #else
          { "perl",    {"perl-interpreter",    {"perl"              }}},
          { "python",  {"python-interpreter",  {"python3", "python" }}},
  #endif
          { "python2", {"python2-interpreter", {"python2", "python" }}},
          { "ruby",    {"ruby-interpreter",    {"ruby"    }}},
          { "shell",   {"shell-interpreter",   {"sh"      }}},
  };

• Search for python3, python is done in script.cpp -> Glib::find_program_in_path(interpreter_path);. It also searches in the system path (see next comment)

      // 2. Search for things in the path if they're there or an absolute
      for (const auto& binname : searchList) {
          auto interpreter_path = Glib::filename_from_utf8(binname);
  
          if (!Glib::path_is_absolute(interpreter_path)) {
              auto found_path = Glib::find_program_in_path(interpreter_path);
              if (!found_path.empty()) {
                  return found_path;
              }
          } else {
              return interpreter_path;
          }

• PATH environment set in AppRun, also includes system path at the end!

  export PATH="${HERE}"/usr/bin/:"${HERE}"/usr/sbin/:"${HERE}"/usr/games/:"${HERE}"/bin/:"${HERE}"/sbin/:"${PATH}"

  while PYTHON_HOME is set as

  export PYTHONHOME="$HERE/usr/"

  hence, in #1306 (closed) Inkscape uses /usr/bin/python3 from system with PYTHONHOME in $HERE/usr/ and not $HERE/usr/bin/python3.8 packed in the AppImage!