New option `org-allow-promoting-top-level-subtree'.
* org.el (org-allow-promoting-top-level-subtree): New option
to allow promoting a top-level subtree.
(org-called-with-limited-levels): New variable, dynamically
bound within the `org-with-limited-levels' macro.
(org-promote): Use the new option to allow promoting a
top-level subtree.
* org-macs.el (org-with-limited-levels): Let-bind
`org-called-interactively-p' to t.
Promoting a top-level subtree can be useful but should not be allowed by
default, as this restructuring is only reversible with M-x undo RET.