Previously on non Windows Platforms the Programm would not get its
icon, as the *.rc is not used on e.g. Linux.
Use an xpm File for non Windows Platforms. Use the convenience macro
from wxWidgets to load the icon in a single call.
The icon was converted using GIMP 3.0.6. To avoid `-Wwrite-strings` the
static char* was converted to a const char*.
- Switched to CMake for configuring and building the doxygen Website.
- Set the minimum requirements to CMake >=3.18.4 and Doxygen >=1.9.1.
- Rewrite the LICENSE.txt and docs README.txt to markdown.
- Add the folder which is used in the README to gitignore.
- Add a custom style to the doxygen website based on the docs webpage.
- Update CI to use the new Doxygen Build process.
Closes#7052