Make the slide property control the workspace scale, so that new workspaces
scale up, and destroyed workspaces scale down. The scale is done horizontally,
and only slightly vertically, as per design direction.
Rework the state tracking mechanism to remove the COLLAPSING state, since there's
no split between sliding out and collapsing anymore. Also remove the corresponding
'collapse-fraction' property from WorkspaceThumbnail.
Make ThumbnailsBox.vfunc_get_preferred_width() consider the slide-position property.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1593>
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