This moves file extension to icon mappings from compile time macros to an
INI config file (/etc/FileIconProvider.ini), so file icons can easily be
customized and extended :^)
I also switched the format from a static file extension (".foo") to
glob-like patterns ("*.foo", using StringUtils::matches()), which allows
us to assign icons to specific exactly matching file names, like many
IDEs do - e.g. "CMakeLists.txt" or ".prettierrc".
