wayland: Port to ServerSideDecorationInterface::preferredMode()
Currently, the wayland server updates the server side decoration mode, which is counter-intuitive, because it doesn't cache the last preferred mode.
With ServerSideDecorationInterface::preferredMode(), it can be simpler.
Taken from !1765 (merged) so I can merge kwayland-server!155 (merged) and make gitlab ci useful.