ui: reject extended clipboard message if not activated

ui: reject extended clipboard message if not activated

The extended clipboard message protocol requires that the client
activate the extension by requesting a psuedo encoding. If this
is not done, then any extended clipboard messages from the client
should be considered invalid and the client dropped.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20240115095119.654271-1-berrange@redhat.com>
(cherry picked from commit 4cba8388968b70fe20e290221dc421c717051fdd)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>