Better detection of potentially malicious bidi text

Better detection of potentially malicious bidi text

* src/bidi.c (bidi_find_first_overridden): Extend to detect more
subtle effects of directional formatting controls, to include
embeddings and isolates.
* src/xdisp.c (Fbidi_find_overridden_directionality): Accept an
additional argument BASE-DIR to specify the base direction of the
paragraphs.

* lisp/international/mule-cmds.el (confusingly-reordered): New
face.
(reorder-starters, reorder-enders): New variables.
(highlight-confusing-reorderings): New command to detect and
highlight suspiciously reordered text.

* test/src/xdisp-tests.el
(xdisp-tests--find-directional-overrides): New test.

* etc/NEWS: Announce the new and improved features.
* etc/tutorials/TUTORIAL.he: Fix embeddings with incorrect
directions.