Refactor file paths

[bqqbarbhg]

The current filename/relative_filename/absolute_filename + raw_ variants does not feel like the most clear solution. It is also duplicated across many different types.

Should probably have some kind of ufbx_file_path structure with some of the following:

• Original relative path
• Original absolute path
• Resolved path relative to the loaded FBX
• Stripped filename without any path