Improvements for Tabulated List mode.

Improvements for Tabulated List mode.

* lisp/emacs-lisp/tabulated-list.el (tabulated-list-format): Accept
additional plist in column descriptors.
(tabulated-list-init-header): Obey it.
(tabulated-list-get-entry): New function.
(tabulated-list-put-tag): Use it.  Use string-width instead of
length.
(tabulated-list--column-number): New function.
(tabulated-list-print): Use it.
(tabulated-list-print-col): New function.  Set
`tabulated-list-column-name' property on each column's text.
(tabulated-list-print-entry): Use it.
(tabulated-list-delete-entry, tabulated-list-set-col): New
functions.
(tabulated-list-sort-column): New command.