(key-sequence): Rework widget to read key binding

(key-sequence): Rework widget to read key binding
using `kbd' syntax.  Use C-q to insert literal key, event, or code.
(widget-key-sequence-default-value): Default value for empty sequence.
(widget-key-sequence-map): New map for reading key binding.  Bind C-q.
(widget-key-sequence-read-event): New command for C-q.
(widget-key-sequence-validate, widget-key-sequence-value-to-internal)
(widget-key-sequence-value-to-external): New functions.