Implement dialog initial focus proposal

This implements the changes proposed here:
https://github.com/whatwg/html/wiki/dialog--initial-focus,-a-proposal#dialog-draft-text

Specifically:
1. Add a parameter to dialog.show() called preventInitialFocus, which
   prevents the dialog focusing steps from running.
2. Make the dialog focusing steps look at sequentially focusable
   elements instead of any focusable element.

There are additional open issues around dialog initial focus listed here:
whatwg#4184 (comment)

TODO add a conformance requirement about autofocus:
whatwg#7709

TODO consider adding a <p class=XXX> for tab trapping:
whatwg#7707

source

@@ -59158,7 +59158,7 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
 
   [<span>CEReactions</span>] attribute boolean <span data-x="dom-dialog-open">open</span>;
   attribute DOMString <span data-x="dom-dialog-returnValue">returnValue</span>;
-  [<span>CEReactions</span>] undefined <span data-x="dom-dialog-show">show</span>();
+  [<span>CEReactions</span>] undefined <span data-x="dom-dialog-show">show</span>(optional boolean preventInitialFocus);
   [<span>CEReactions</span>] undefined <span data-x="dom-dialog-showModal">showModal</span>();
   [<span>CEReactions</span>] undefined <span data-x="dom-dialog-close">close</span>(optional DOMString returnValue);
 };</code></pre>
@@ -59243,8 +59243,9 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
 
   <div w-nodev>
 
-  <p>When the <dfn method for="HTMLDialogElement"><code data-x="dom-dialog-show">show()</code></dfn>
-  method is invoked, the user agent must run the following steps:</p>
+  <p>When the <dfn method for="HTMLDialogElement"><code
+  data-x="dom-dialog-show">show(preventInitialFocus)</code></dfn> method is invoked, the user agent
+  must run the following steps:</p>
 
   <ol>
    <li><p>If the element already has an <code data-x="attr-dialog-open">open</code> attribute, then
@@ -59256,7 +59257,8 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    <li><p>Set the <code>dialog</code> element's <span>previously focused element</span> to the
    <span>focused</span> element.</p></li>
 
-   <li><p>Run the <span>dialog focusing steps</span> for the <code>dialog</code> element.</p></li>
+   <li><p>If <var>preventInitialFocus</var> flag is not present or is set to false, run the
+   <span>dialog focusing steps</span> for the <code>dialog</code> element.</p></li>
   </ol>
 
   <p>When the <dfn method for="HTMLDialogElement"><code
@@ -76934,10 +76936,21 @@ END:VCARD</pre>
 
     <ol>
      <li>
-      <p>Let <var>focusable area</var> be <var>child</var>, if <var>child</var> is a
-      <span>focusable area</span>; otherwise let <var>focusable area</var> be the result of <span
-      data-x="get the focusable area">getting the focusable area</span> for <var>child</var> given
-      <var>focus trigger</var>.</p>
+      <p>Let <var>focusable area</var> be null.</p>
+
+      <ol>
+       <li><p>If <var>focus target</var> is a <code>dialog</code> element and <var>child</var> is
+       <span>sequentially focusable</span>, let <var>focusable area</var> be
+       <var>child</var>.</p></li>
+
+       <li><p>Otherwise, if <var>focus target</var> is not a <code>dialog</code> element and
+       <var>child</var> is a <span>focusable area</span>, let <var>focusable area</var> be
+       <var>child</var>.</p></li>
+
+       <li><p>Otherwise, let <var>focusable area</var> be the result of <span data-x="get the
+       focusable area">getting the focusable area</span> for <var>child</var> given <var>focus
+       trigger</var>.</p></li>
+      </ol>
 
       <p class="note">This step can end up recursing, i.e., the <span>get the focusable area</span>
       steps might return the <span>focus delegate</span> of <var>child</var>.</p>