Fix bug #16043 with crashes when displaying new bidi control characters.

Fix bug #16043 with crashes when displaying new bidi control characters.

 src/bidi.c (bidi_get_type, bidi_get_category): Handle the isolate
 directional control characters.  Update type and category
 determination according to the UBA from Unicode v6.3.
 (bidi_category_t): New category EXPLICIT_FORMATTING.
 src/dispextern.h (bidi_type_t): Update to include new bidirectional
 properties introduced with Unicode v6.3.

 admin/unidata/unidata-gen.el (unidata-prop-alist): Update bidi-class
 to include the new isolate-related classes introduced with Unicode
 v6.3.
 (unidata-encode-val): Accept an additional optional argument, a
 warning message to emit when UnicodeData.txt defines bidi-class
 values that are not in unidata-prop-alist.  Add a comment
 explaining what should maintainers do if/when such a warning ever
 appears.
 (unidata-gen-table): Call unidata-encode-val with 3rd arg non-nil
 when generating uni-bidi.el.