[kwin-wayland] Add support for FullscreenShell

The FullscreenShell is a Wayland protocol provided by Weston to have
exactly one surface per output. This is exactly what KWin needs. So
in case the Wayland server we connect to provides the FullscreenShell
we prefer it over the normal Shell and mapping our surface as fullscreen.

The protocol is not yet part of wayland-client library, so the header
and source file needs to be generated. This is done during the build
process using the external tool wayland-scanner. The protocol
description is copied from the Westion 1.5 sources.

REVIEW: 119839