Fix for bug18584. Add new properties for the save message dialog. R=jefft

mailnews/compose/resources/locale/en-US/composeMsgs.properties

@@ -169,3 +169,10 @@ noIdentities=You don't have any email identities yet.  Create one with the Accou
 
 ## @name NS_MSG_ATTACHMENT_TYPE_MISMATCH
 12552=<Image Signature Omitted>
+
+## Strings use for the save message dialog shown when the user close a message compose window
+saveDlogTitle=Save Message
+saveDlogMessage=Message has not been Sent. Do you want to save the message in the Drafts Folder?
+saveDlogSaveBtn=Save
+saveDlogDontSaveBtn=Don't Save
+saveDlogCancelBtn=Cancel