improve org mode, add new functions
improve org mode, add new functions

* simplify org_next/prev_heading:
  - always return valid offset
  - level = 0 if not heading found
  - pointer to level can be NULL
* add new functions:
  - do_outline_next_vsible_heading      (C-c C-n)
  - do_outline_previous_vsible_heading  (C-c C-p)
  - do_outline_up_heading               (C-c C-u)
  - do_org_backward_same_level          (C-c C-b)
  - do_org_forward_same_level           (C-c C-f)
  - do_org_goto                         (C-c C-j)
  - org-insert-heading-respect-content  (C-enter)
  - org-insert-todo-heading-respect-content
* add flags in do_org_insert_heading

dmacvicar/qemacs,dmacvicar/qemacs,dmacvicar/qemacs,dmacvicar/qemacs