seat_get_active_child is used to get the active tiling child in a few
places, such as outputs getting their active workspace and
tabbed/stacked containers getting their visible child. When a workspace
uses a tabbed or stacked layout and contains a focused floating view,
calling seat_get_active_child on the workspace would incorrectly return
the floating view. This changes it so it will return the tiling child.
This fixes the following bug:
* Create layout T[view view] then float one of the views
* Attempt to click the tiling view to give it focus - it wouldn't work
because seat_get_active_child would return the floating view
48bc15e758