fix 368822 respect mail.biff.alert.show_preview and don't fetch preview text if it's not set, sr=mscott

mailnews/base/resources/content/mailWidgets.xml

@@ -2037,6 +2037,8 @@
             // skip servers, Trash and Junk folders
             if (!aFolder || aFolder.isServer || aFolder.getFlag(kMsgPopupFolderFlagJunk) || aFolder.getFlag(kMsgPopupFolderFlagTrash))
               return false;
+            var pref = Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefBranch);         
+            var showPreviewText = pref.getBoolPref("mail.biff.alert.show_preview");
             var folderArray = new Array;
             if (aFolder.flags  & MSG_FOLDER_FLAG_VIRTUAL)
             {
@@ -2069,6 +2071,9 @@
               if (!numMsgKeys.value)
                 continue;
 
+
+              if (showPreviewText)
+              {
                 // fetchMsgPreviewText forces the previewText property to get generated
                 // for each of the message keys.
                 try {
@@ -2082,7 +2087,7 @@
                   aFolder.setMsgDatabase(null);
                   continue; 
                 }
-
+              }
               // if fetching the preview text is going to be an asynch operation and the caller
               // is set up to handle that fact, then don't bother filling in any of the fields since 
               // we'll have to do this all over again when the fetch for the preview text completes.