workspacesView: Add primary view to workspaces display

workspacesView: Add primary view to workspaces display

Boy, does this commit feel good.

While the workspaces view on the primary monitor *appears* as part of
the overall overview hierarchy, this hasn't actually been the case
until now. We synchronized its size and (stage) position to match
the workspaces display, but actually kept in a separate layer for
the transitions to and from the overview.

But now that the new layout manager slides out completely during the
overview transitions, the workspaces display starts out covering the
entire work area, which is exactly what we need for the transition.

So finally stop faking it, and actually make the primary workspaces
view a child of the workspaces display.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1624>