Make accents zero width (#1033)

* Make accents zero width

Make the width of an accented character equal to the width of the character,
by making accents zero width.  In particular, fixes #1028 (`\ddot\imath`).

This involved reworking all of the accent offset machinery, in particular
skew and accent-hungarian.  A bit cleaner now.

Also added support for the new width character metrics in symbolNode.

* Update AccentsText test

* Fix comment

static/katex.less

@@ -554,13 +554,9 @@
             text-align: center;
         }
 
-        .accent-body > span {
+        .accent-body {
             width: 0;
-        }
-
-        .accent-body.accent-hungarian > span {
-            position: relative;
-            left: 0.250em; // width of space
+            position: relative; // so that 'left' can shift the accent
         }
     }