Simplify FileNode

[tshikaboom]

This is more or less me writing down what #19 has made me remark about the state of class FileNode.

• Simplify the class definition and its parsing (the header itself is one uint32_t only, use that to infer the FileNode's properties afterwards, and not explicitly store them as separate members
• Move skip_node() to a place better suited for what this function does, like FileNodeListFragment