only show action button on last of multiple notification popups

inspect notification list before display and modify notification popup
windows to remove action buttons (such as "Go To Base" or "Allocate
Research") from all but the last of a sequence of similar popups.

This would solve issue 416
http://openxcom.org/bugs/openxcom/issues/416

In addition, empty "NewPossibleResearch" popup notifications are no
longer shown.