Correctly handle bullets with two spaces.
* org-list.el (org-list-bullet-string): New function returning bullet
concatenated with an appropriate number of white spaces.
* org-list.el (org-list-insert-item-generic): Insert the right bullet,
with help of `org-list-bullet-string'.
* org-list.el (org-indent-item-tree): Use `org-list-bullet-string'.
* org-list.el (org-fix-bullet-type): Use `org-list-bullet-string'.
* org-list.el (org-toggle-checkbox): send an error when
`org-toggle-checkbox' is trying to insert a checkbox at a
description item.
* org-list.el (org-item-re): modified regexp so it can catch correct
number of white space before item body.
* org-list.el (org-list-at-regexp-after-bullet-p): Take into
consideration new `org-item-re'.