* register.el (register-preview-delay)

* register.el (register-preview-delay)
(register-preview-functions): New variables.
(register-read-with-preview, register-preview)
(register-describe-oneline): New functions.
(point-to-register, window-configuration-to-register)
(frame-configuration-to-register, jump-to-register)
(number-to-register, view-register, insert-register)
(copy-to-register, append-to-register, prepend-to-register)
(copy-rectangle-to-register): Use register-read-with-preview to
read register.