separate option editor_skip_proofs, to avoid accidental change of preferences for skip_proofs, which would invalidate batch builds;

more ambitious Goal.skip_proofs: covers Goal.prove forms as well, and do not insist in quick_and_dirty (for the sake of Isabelle/jEdit);

clarified Skip_Proof.cheat_tac: more standard tactic; clarified Method.cheating: check quick_and_dirty when it is actually applied;

tuned signature and module arrangement;

tuned;

tuned GUI;