Rename gtk_tree_path_new_from_node() to tree_path_new_from_node()

Rename gtk_tree_path_new_from_node() to tree_path_new_from_node()

This is a function internal to the file system model; let's not pollute the gtk_tree_path namespace.

Also, make the 'i' variable into 'r' as it refers to a row index, not a file-array index (for
consistency with the docs and the rest of the code).

Signed-off-by: Federico Mena Quintero <federico@gnome.org>