desktop-shell: add set_panel_position to help place views onscreen

desktop-shell: add set_panel_position to help place views onscreen

Panels are always assumed to be on the top edge of the output. If this
is not the case views will be placed under the panel, wherever it is,
and maximize doesn't use the correct space allocated for views.

By telling the server on which edge the panel is located, it can
correctly calculate where to put new views and how big maximized views
should be.

[Pekka Paalanen: the user of this protocol so far is Maynard.]

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>