Hide the mousewheel prefs panel on Macintosh, since we do not have

mousewheel scrolling implemented there.  r=ben.

xpfe/components/prefwindow/resources/content/pref.xul

@@ -40,6 +40,13 @@
           var element = document.getElementById( "cookiesCell" );
           element.setAttribute( "value", element.getAttribute( "valueWithImages" ) );
         }      
+
+      if (top.window.navigator.platform == "Macintosh") {
+        var advancedChildren = document.getElementById("advancedChildren");
+        var mousewheelItem = document.getElementById("mousewheelItem");
+        advancedChildren.removeChild(mousewheelItem);
+      }
+
       // Test for presence of "winhooks" component.
       var showWinhooks = false;
       try {

xpfe/components/prefwindow/resources/content/preftree.xul

@@ -186,7 +186,7 @@
       <treerow>
         <treecell class="treecell-indent" url="chrome://communicator/content/pref/pref-advanced.xul" value="&advance.label;"/> 
       </treerow>
-      <treechildren>
+      <treechildren id="advancedChildren">
         <treeitem>
           <treerow>
             <treecell class="treecell-indent" url="chrome://communicator/content/pref/pref-cookies.xul"
@@ -209,7 +209,7 @@
             <treecell class="treecell-indent" url="chrome://communicator/content/pref/pref-proxies.xul" value="&proxies.label;"/> 
           </treerow>
         </treeitem>
-        <treeitem>
+        <treeitem id="mousewheelItem">
           <treerow>
             <treecell class="treecell-indent" url="chrome://communicator/content/pref/pref-mousewheel.xul" value="&mousewheel.label;"/>
           </treerow>