place: Center the first window

When opening the first window on a workspace, we aren't constrained by any other windows: We have the whole work area available to pick a sane placement.

Now the current position isn't exactly insane, but a lone window does look more balanced when centered.

As a bonus, "center tile-rect in area" will now actually center the tile-rect in the area :-)

See https://pagure.io/fedora-workstation/issue/79 for the context that triggered the patch.