workspacesView: Center active workspace in allocation

workspacesView: Center active workspace in allocation

When vertically snapping, WorkspacesView currently allocates workspaces
side-by-side, then applies an extra step of translation to center to
the active workspace. This extra step, however, gets in our way because
now we need tighter control of the workspaces positions in allocation,
in order to properly interpolate them.

Move the translation of workspaces to the allocation code itself, and
remove the extra translation step.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1613>