restore panel when screen geometry changes

the panel size and position is saved per-screen resolution
so at different resolutions (or different screens) the panel always goes
back where it was with the size it had.
this should give a more deterministic behavior to the panel
when resizing the screen (like virualbox) or docking to
an external docking styation

BUG:368741
reviewed-by: David Edmundson <david@davidedmundson.co.uk>