workspace: Make computeLayout() return a layout object

workspace: Make computeLayout() return a layout object

Start cleaning up the whole mess around the layout object a bit and
return a new object in the LayoutStrategies computeLayout()
implementation. This object is supposed to be opaque to the API user and
will only be passed to the layout strategy.

For now, keep setting a few things on that object from outside, we'll
clean that up later.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1617>