toolbox: Add file descriptor
toolbox: Improve doc comments
toolbox: Add dummy for slice
toolbox: Improve io
lip: Add accessor to string

Added accessor function to retrieve the backing Toolbox_String object of an
MKX_String object.
toolbox: Update copyright notices
toolbox: Improve doc comments
lip: Update copyright notices
lip: Improve doc comments
math: Update copyright notices
editor: Update copyright notices