Rename adaptive-wrap-prefix to adaptive-wrap.

The old name overflowed the column in list-packages.
Add adaptive-wrap-extra-indent.
* adaptive-wrap/adaptive-wrap.el (adaptive-wrap-extra-indent): New var.
(adaptive-wrap-fill-context-prefix): New function.
(adaptive-wrap-prefix-function): Use it.
(adaptive-wrap-prefix-mode): Add to visual-line custom group.
Clean up copyright notices.
* adaptive-wrap.el (adaptive-wrap-extra-indent): Fix buffer-localness.
Reported by Jonathan Kotta <jpkotta@gmail.com>.
Fix error during redisplay: (wrong-type-argument stringp nil)
* adaptive-wrap.el (menu-bar-options-menu): Add checkbox for Adaptive Wrap
to the Line Wrapping submenu.
(adaptive-wrap-unload-function): New function.
* adaptive-wrap.el: Require easymenu (bug#14974).
* adaptive-wrap.el: Fix bug#14974 by using define-key-after
instead of easy-menu-add-item.
(adaptive-wrap-unload-function): Remove.
* adaptive-wrap.el (adaptive-wrap-mode): Move after font-lock (bug#15155).