workspaceThumbnail: Allocate final thumbnail sizes
ThumbnailsBox currently allocates each workspace thumbnail using their
porthole size, and scales them down using scale-x and scale-y. This is
slightly problematic since it doesn't allow for properly styling these
thumbnails through CSS.
Rework ThumbnailsBox to allocate workspace thumbnails at their actual
sizes, and scale down the '_contents' actor inside WorkspaceThumbnail.