Bug 1061450 - Use customized dialog for delete APN waring r=eragonj

https://bugzilla.mozilla.org/show_bug.cgi?id=1061450
The code you suggested behaves differently. We can only remove the listeners and resolve the promise when users click on either the ok button or the cancel button. Removing the listener unconditionally makes the the dialog disappearing and return false when users click on any place other the ok button.