Allow moving members of editable-list widget, via delete+insert

Allow moving members of editable-list widget, via delete+insert

* etc/NEWS (Widget): Announce the feature (bug#6419).
* lisp/wid-edit.el (widget-editable-list-delete-at): Save into a new
widget property, :last-deleted, the WIDGET to be deleted.  Add
docstring.
(widget-editable-list-insert-before): If there is a recently deleted
child for the editable list, insert that one, instead of a new default
widget.  Add docstring.
(insert-button widget): Make :help-echo a function to avoid the
help-echo string become too long.
(delete-button widget): Tweak the :help-echo string, to document this
behavior.

* test/lisp/wid-edit-tests.el (widget-test-moving-editable-list-item):
Test the feature.