Structure editing: Improved item/headline conversion

Structure editing: Improved item/headline conversion

    The conversion commands `C-c -' and `C-c *' are now better
    behaved and therefore more useful, I hope.

    If there is an active region, these commands will act on the
    region, otherwise on the current line.

    - C-c - :: This command turns headings or normal lines into
         items, or items into normal lines.  When there is a
         region, everything depends on the first line of the
         region:
         - if it is a item, turn all items in the region into
           normal lines.
         - if it is a headline, turn all headlines in the region
           into items.
         - if it is a normal line, turn all lines into items.
         - special case: if there is no active region and the
           current line is an item, cycle the bullet type of the
           current list.
    - C-c * :: This command turns items and normal lines into
         headings, or headings into normal lines.  When there is
         a region, everything depends on the first line of the
         region:
         - if it is a item, turn all items in the region into
           headlines.
         - if it is a headline, turn all headlines in the region
           into normal lines.
         - if it is a normal line, turn all lines into headlines.