seat_get_active_child is used for tabbed and stacked containers to get
the active child. The previous implementation used seat_get_focus_inactive
then ascended the tree to the child of the tabbed/stacked container, but
this fails when the workspace itself is stacked or tabbed and the most
recently active child is floating.
The new implementation takes a more simple approach, where it directly
scans the focus stack for the first immediate child which isn't the
floating container.
Fixes#2098.
57e78414fa