This adds support for the KDE-specific server-side decorations protocol.
Server-side decoration are required by all non-embedded video window
plugins in VLC. Inventing VLC-specific decoration just for this purpose
would not make sense.

Any display server can implement the protocol. But in practice, this
probably does not work with display servers other than Kwin. (It
definitely does not work with Weston; I have not tried other servers.)