* allout.el Summary: Change so yank of distinctive-bullet items preserves

* allout.el Summary: Change so yank of distinctive-bullet items preserves
the existing header prefix, rebulleting it if necessary, rather than
replacing it.  This is necessary for proper operation of cooperative addons
like allout-widgets.

(allout-make-topic-prefix) (allout-rebullet-heading): Change SOLICIT arg to
INSTEAD, and interpret additionally a string value as alternate bullet to
be used, instead of prompting the user for a bullet character.