Window decoration scrolling: don't focus sibling

Window decoration scrolling: don't focus sibling

The current behaviour is buggy in the following layout:
T [ A* V [ B C ] ], where the focus stack in V is B > C.
When the user scrolls down, focus correctly moves to B but if the user
scrolls down again the whole vertical container is focused.

This happens because 'bool scroll_next_possible' is false but
con_activate is called on the tabbed container's sibling - the vertical
container.