ScalesList-related functions should be moved into ggproto object

[wch]

There are some functions that operate on ScalesList objects. They should be turned into methods.

[paleolimbot]

For reference, these functions all (I think) live in R/scales-.r, and are scales_train_df(), scales_map_df(), scales_transform_df(), scales_add_defaults(), and scales_add_missing().