Add a close button to Popup's title bar

I thought a close button might be useful, especially for dialog popups with no buttons. I didn't implement any default config logic in PopupManager; I just modified buildConfig to modify a copy of the passed config instead of a blank object. The commits are relatively minor, and the formatting on the close button looks OK. However, I am worried about what will happen if the font size for the close button changes. Maybe someone who knows more about styling can tidy the close button up.