Improve Isearch in pcre-mode

- Use isearch-mode-hook and isearch-mode-end-hook to add and remove the
  custom search callback

- Cache converted regexps to avoid re-computing on each search

- Signal an error on regexps with a trailing backslash, which prevents
  isearch from prematurely moving ahead to a literal backslash in the
  buffer.  Probably this case should be caught in the main
  rxt-pcre-to-elisp function.
Re-implement pcre-query-replace-regexp using pcre-mode

If the new pre-mode works well this function will probably go away at
some point.
Tweaks to pcre-mode comments & prompts