Allow plasmashell interface to set struts

The motivation behind this is that it's needed by plasma-desktop!1290 (merged); when the panel is floating the strut should not be the same size as the window, but rather always be the size the panel will have as soon as it de-floates.

See also: libraries/plasma-wayland-protocols!53 (closed) and https://invent.kde.org/frameworks/kwayland/-/merge_requests/95 for the other two pieces of the MR