Factor decision-to-close into an extension

[gabrielschulhof]

The default popup should not close in response to any event except the click upon the screen. The various events in response to which it auto-closes should be added as extensions.

[gabrielschulhof]

This would make it possible to implement the loader via an undismissible popup thereby satisfying the feature request that a page should be inaccessible while another page is loading.