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, content free "NewPossibleResearch" popup notifications (that is, empty popups with no text and just an 'ok' button) are no longer shown.

There is a minor reordering of popups if a completed research item provides a new item to manufacture.  Before, newly manufacturable items were shown immediately after their respective completed research item.  Now, all completed research items are shown and then all newly manufacturable items are shown.  If the previous ordering is preferred, I can modify the code to preserve it, but the "more" "more" "allocate research"  "more" "more" "allocate manufacture" button click sequence seems to make more sense than the original order, which would be "more" "more" "more" "more" "allocate research" "allocate manufacture".