Remove 'double' UnderlineStyle (#91)

The `double` UnderlineStyle was added in anticipation of a planned Windows feature that never came to be. There are no other known uses, so remove it.

Closes #89

Author: dandclark

index.html

@@ -1189,7 +1189,7 @@ <h3>TextUpdateEvent</h3>
         </section>
         <section>
             <h3>TextFormatUpdateEvent</h3>
-            <pre class="idl"><xmp>enum UnderlineStyle { "none", "solid", "double", "dotted", "dashed", "wavy" };
+            <pre class="idl"><xmp>enum UnderlineStyle { "none", "solid", "dotted", "dashed", "wavy" };
 enum UnderlineThickness { "none", "thin", "thick" };
 
 dictionary TextFormatInit {