overview: Get rid of panel ghost actor

overview: Get rid of panel ghost actor

Right now we use a ClutterClone ghost of the panel simply to add some
spacing to the top monitor edge.

We can remove the panelGhost by adjusting the allocations of all the
parts of the overview ourselves inside ControlsManagers vfunc_allocate,
this should also work with extensions that move the panel somewhere
else.

This makes the initial relayout of the overview significantly faster,
because we now no longer have to relayout the whole panel in the
process.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1755>