Fix list item parsing and headline token lvls

As headlines are always lvl (indent) 0 I thought it would be clever to abuse
the lvl field to store the headline lvl. Well, here we are - it wasn't clever.

List items only end when their parent ends or they run into something that's
not indented enough - everything else becomes part of the list item. Abusing
the token.lvl field (indent) for the headline lvl means headlines look indented
to the list item parsing logic - i.e. they become part of the list item if the
headline has a high enough lvl. That should never happen - so let's get rid of
the hack and (re-)calculate the headline lvl when we need it.

org/testdata/misc.org

@@ -140,6 +140,10 @@ When inserting an image link like [[/home/amos/Pictures/Screenshots/img-2017-09-
 *** index out of range in explicit line break parsing
 0\\ 
 
+*** list items don't end on child headline
+- a list item
+**** followed by a child headline
+- followed by another list item
 * Footnotes
 
 [fn:1] a footnote /with/ *markup*