UnlockDialog: allow going back to the lock screen by pressing escape

https://bugzilla.gnome.org/show_bug.cgi?id=681143

js/ui/unlockDialog.js

@@ -143,6 +143,7 @@ const UnlockDialog = new Lang.Class({
                            action: Lang.bind(this, this._doUnlock),
                            default: true };
         this.setButtons([this._okButton]);
+        this.setActionKey(Clutter.KEY_Escape, Lang.bind(this, this._escape));
 
         this._updateOkButton(false);
         this._reset();
@@ -202,6 +203,10 @@ const UnlockDialog = new Lang.Class({
         this.emit('failed');
     },
 
+    _escape: function() {
+        this._onVerificationFailed();
+    },
+
     _otherUserClicked: function(button, event) {
         this._userManager.goto_login_session();