make threadpane pause 500 milliseconds before selecting a msg r=blaker, sr=jst, a=asa 163946

mailnews/base/resources/content/threadPane.xul

@@ -31,7 +31,7 @@ Rights Reserved.
 
 <script src="chrome://messenger/content/threadPane.js"/>
 
-<tree id="threadTree" flex="1" enableColumnDrag="true" class="plain" disableKeyNavigation="true"
+<tree id="threadTree" flex="1" enableColumnDrag="true" _selectDelay="500" class="plain" disableKeyNavigation="true"
     onkeypress="ThreadPaneKeyPress(event);"
     onselect="ThreadPaneSelectionChanged();">
   <treecols>