Make line widget handles objects with methods. Handle widget height better

Closes #1147

Widget handles now have .clear() and .changed() methods. The second informs CodeMirror
that the widget changed, and causes it to recompute the line height for the widget's
line.

Fixes some sloppiness in the tracking of widget heights.

Author: marijnh

doc/manual.html

@@ -978,12 +978,15 @@ <h2 id="api">Programming API</h2>
       CodeMirror-specific CSS classes, and those classes might in some
       cases affect it. This method returns an object that represents
       the widget placement. It'll have a <code>line</code> property
-      pointing at the line handle that it is associated with, and it
-      can be passed to <code>removeLineWidget</code> to remove the
-      widget.</dd>
-
-      <dt id="removeLineWidget"><code>removeLineWidget(widget)</code></dt>
-      <dd>Removes the given line widget.</dd>