Fix for #7462: in sq 8948 (and before) pane spliters show themselves and are active even when window is collapsed

Hi,

I uploaded Morphic-kb.316 to The Inbox to fix
http://bugs.squeak.org/view.php?id=7462
The fix adds a check to SystemWindow >> activate, to avoid sending
#addPaneSplitters if the window isCollapsed.

Balázs