loginDialog: s/button-press-event/key-press-event/

A bug got introduced when moving the login dialog away from modal
dialog, such that it listens for escape key presses in a mouse
event handler instead of a keyboard event handler.

This commit fixes that code to correctly listen for key-press-event
instead of button-press-event.

https://bugzilla.gnome.org/show_bug.cgi?id=702308

js/gdm/loginDialog.js

@@ -470,7 +470,7 @@ const LoginDialog = new Lang.Class({
         this._promptBox = new St.BoxLayout({ style_class: 'login-dialog-prompt-layout',
                                              vertical: true });
 
-        this._promptBox.connect('button-press-event',
+        this._promptBox.connect('key-press-event',
                                  Lang.bind(this, function(actor, event) {
                                     if (event.get_key_symbol() == Clutter.KEY_Escape) {
                                         this.cancel();