workspaceThumbnails: Expand workspaces before scale-in animation

When the original animation was implemented, workspaces would only ever be added at the end. We therefore got away with not having a separate EXPANDING stage corresponding to the existing COLLAPSING one when animating out.

Since support for creating in-between workspaces via DND was added, this is no longer the case. And now that the thumbnails are centered, the jump is quite noticeable.

Address this by adding new transitional states, so that we can expand new thumbnails before scaling them in.