(fix) properly cleanup the Trash folder after empty'ing

UI/WebServerResources/js/Mailer/MailboxesController.js

@@ -138,7 +138,10 @@
 
     function emptyTrashFolder(folder) {
       folder.$emptyTrash().then(function() {
-        // Success
+        // Success - remove all messages from the mailbox
+        folder.$messages = [];
+        folder.uidsMap = {};
+        folder.unseenCount = 0;
       }, function(error) {
         Dialog.alert(l('Warning'), error);
       });