131762 - can open multiple occurrences of download manager. also adds a pref in Navigator > Downloads to set the behavior when starting a download (open DM, open progress dialog, open nothing). r=hewitt sr=ben a=asa

xpfe/components/download-manager/public/nsIProgressDialog.idl

@@ -61,6 +61,12 @@ interface nsIProgressDialog : nsIDownload {
    * open yet, or has been closed.
    */   
    attribute nsIDOMWindow dialog;
+   
+  /**
+   * Whether or not to cancel the download when the progress dialog is closed.
+   * If set, the Cancel button will also be hidden.
+   */
+   attribute boolean cancelOnClose;
 };