Add one-off search element to XBL popup

Fixes #296.

chrome/content/binding.xml

@@ -17,6 +17,11 @@
       <xul:richlistbox anonid="richlistbox" class="autocomplete-richlistbox" flex="1"/>
       <xul:hbox anonid="footer">
         <children/>
+        <xul:vbox anonid="one-off-search-buttons"
+                  class="search-one-offs"
+                  compact="true"
+                  includecurrentengine="true"
+                  flex="1"/>
       </xul:hbox>
     </content>
   </binding>