Imprement "close" button on helper line.

[sugi]

On multi select mode, some user report he is puzzled how to close choice popup window. Certainly user can close the popup by clicking outside of popup. However, I actually guess that normal user is not familior with that UI.

I propose to add a "close" button on popup. This patch add the button on helper line.

[davidda]

+1 I also feel my users may wonder about that. I think this is a good addition and I'm already using it. However, I found that the reset button does not appear anymore with this code. I don't need it anyways, but this could be an issue in your pull request.

[grahamroy]

+1 this is a good idea.