workspacesView: Decrease workspaces size on external monitors

workspacesView: Decrease workspaces size on external monitors

Now that the backgrounds was moved into workspaces, the fullscreen
views on secondary monitors are visually inconsistent with the
primary view, as there's no dash or search entry that reduces the
available height and allows adjacent workspaces to peek in.

Address this by adding padding above and below the view, so that
it is limited to 70% of the available height.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1735>