transfer package: add test to prevent trying to make cterms from open terms

transfer package: more flexible handling of equality relations using is_equality predicate

ensured that rewr_conv rule t = "t == u" literally not just modulo beta-eta

new theorems

incorporated constant chars into instantiation proof for enum; tuned proofs for properties on enum of chars; swapped theory dependency of Enum.thy and String.thy

close code theorems explicitly after preprocessing

tuned proofs;

further attempts to cope with large files via option jedit_text_overview_limit;

more detailed Prover IDE NEWS;

recovered explicit error message, which was lost in b8570ea1ce25;