Release 0.13.0

• version 0.13.0

• Change the code to C++20 only
• First trivial C++ refactorings (introduction of namespaces, fewer
  preprocessor magic, idiomatic data structures)
• Remove the hand written double linked lists and use standard containers
• big internal rework
• the server console now only supports '=' to assign option values (this is due
  to a refactoring, supporting the old behavior is too much of a hassle, as I
  don't expect that anyone on this planet uses the console)
• Improve the build helper script
  • Always delete and recreate an existing build directory
• [bugfix/pr #24] Autoconf library detection bugfix
• [pr #27] The project can be build on OpenSuse
• fix to memory related bug in the parser and parser test code (bffb51 and
  fd5f42).
• Remove usage of deprecated glib functions.
• [bug #21] Determine ressource paths at run time relative to the executable
  file
• [bug #53] Fix use-after-free memory corruption error.