Instead of delegating it to ViewSelector, make the transition to and from
overview ease the main state adjustment.
This commit temporarily breaks these animations, but on the other hand
introduces an important feature: ViewSelector is always allocated to the
actual size. This will finally allow for adding WorkspacesView as a child
of WorkspacesDisplay, and finally remove the actual geometry hack, which
is what next commit is about.
This commit also effectively reverts b64103efc.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1624>
b6337a7bf1