Display some character widget values in a more user-friendly way

Display some character widget values in a more user-friendly way

* lisp/wid-edit.el (widget-character--escape-sequences-alist): New
variable.
(widget-character--change-character-display): New function.  Use the new
variable.
(widget-character-notify): New function, to keep track of the changes
in the character widget, and display characters like tab,
newline and spaces better.
(character widget): Use widget-character-notify as the notify
function.  Use widget-character--change-character-display for the
internal representation of value (bug#15925).