workspace: Move window previews to a child container

workspace: Move window previews to a child container

Currently, Workspace is a single actor that contains both the background,
and all window previews, and is managed by WorkspaceLayout. In the future,
this concentrating aspect of it will bite us; we want the window previews
to be clipped to the allocation, but not the background, since it will
have shadows.

Make Workspace subclass St.Widget with a ClutterBinLayout as layout manager,
and move window previews to a child actor. To reduce the impact of this
extra actor, it's a ClutterActor instead of a StWidget, and the spacing is
still set on Workspace itself.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1691>