revise split transformations
Provide nine transformations: split_(goal|all|premise)_(full|right|wp). split_*_full splits as far as it can, split_*_right produces linear number of subformulas, split_*_wp stops at the "stop_split" label. The name "split_goal" is kept for compatibility with older session files and denotes the same transformation as "split_goal_wp". Thanks to Johannes Kanig for the suggestion.