Add IE-specific 'unselectable' attribute

I ended up needing to add this attribute to work around a common IE bug, that clicking on a scrollbar gives focus to the scroll container. I needed to set it as MUST_USE_ATTRIBUTE or the attribute would not be passed through by React successfully.

remove trailing spaces

src/browser/ui/dom/HTMLDOMPropertyConfig.js

@@ -179,7 +179,9 @@ var HTMLDOMPropertyConfig = {
     itemID: MUST_USE_ATTRIBUTE,
     itemRef: MUST_USE_ATTRIBUTE,
     // property is supported for OpenGraph in meta tags.
-    property: null
+    property: null,
+    // IE-only attribute that controls focus behavior
+    unselectable: MUST_USE_ATTRIBUTE
   },
   DOMAttributeNames: {
     acceptCharset: 'accept-charset',