Fix file path resolution to be portable

[wi-re]

In the previous version of the code resources were loaded from either a hardcoded path (the source directory) or the working directory (if a CMake flag was set). This meant that code compiled with a hardcoded string is not portable to other systems and that the working directory version does not work out of the box for simply launching the executable from its own folder. This PR fixes this.

The fix involves creating a config file that CMake fills with the source and build paths and then adding a path resolver (under src/PathFinder.h) that looks at those two paths, which are not portable, but also looks at the current working directory and the directory where the binary is located, which should cover most cases. The resolve also handles paths in the home directory ('~').

To use the resolver:

#include "PathFinder.h"
//...

std::filesystem::path file = resolveFile("resources/colormaps.txt");