Add API in QFileInfo to check if a file is a special file

The std::filesystem library lets you check whether a file is a special Socket, FIFO, Block or Character file (https://en.cppreference.com/w/cpp/filesystem/file_type) which is something that we seem to be lacking in QFileInfo.

My proposal is to either add a new isSpecialFile() getter, or to add an enum QFileInfo::Type, together with a QFileInfo::Type type() const getter, to check the file type.