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.html

@@ -72,9 +72,15 @@
 </li>
 <li><a href="#headline-30">index out of range in explicit line break parsing</a>
 </li>
+<li><a href="#headline-31">list items don&#39;t end on child headline</a>
+<ul>
+<li><a href="#headline-32">followed by a child headline</a>
+</li>
 </ul>
 </li>
-<li><a href="#headline-31">Footnotes</a>
+</ul>
+</li>
+<li><a href="#headline-33">Footnotes</a>
 </li>
 </ul>
 </nav>
@@ -550,10 +556,34 @@ index out of range in explicit line break parsing
 <p>0\\ </p>
 </div>
 </div>
+<div id="outline-container-headline-31" class="outline-4">
+<h4 id="headline-31">
+list items don&#39;t end on child headline
+</h4>
+<div id="outline-text-headline-31" class="outline-text-4">
+<ul>
+<li>
+<p>a list item</p>
+</li>
+</ul>
+<div id="outline-container-headline-32" class="outline-5">
+<h5 id="headline-32">
+followed by a child headline
+</h5>
+<div id="outline-text-headline-32" class="outline-text-5">
+<ul>
+<li>
+<p>followed by another list item</p>
+</li>
+</ul>
 </div>
 </div>
-<div id="outline-container-headline-31" class="outline-2">
-<h2 id="headline-31">
+</div>
+</div>
+</div>
+</div>
+<div id="outline-container-headline-33" class="outline-2">
+<h2 id="headline-33">
 Footnotes
 </h2>
 </div>

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*

org/testdata/misc.pretty_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*