New allowed value 'start-level for `org-loop-over-headlines-in-active-region'.

New allowed value 'start-level for `org-loop-over-headlines-in-active-region'.

* org.el (org-scan-tags): New parameter `start-level' to
scan only through headlines of that level.
(org-map-entries): New allowed value `region-start-level' for
the `scope' parameter, to allow scanning through headlines of
the same level than the first headline in the region.
(org-loop-over-headlines-in-active-region): New allowed value
'start-level.

This change gives more flexibility when looping over the active
region for commands like `org-schedule', `org-deadline', etc.
By setting `org-loop-over-headlines-in-active-region' to
̀start-level', those command will act upon headlines that are
of the same level than the first one in the region.