vnc: track LED state separately

Piggy-backing on the modifier state array made it difficult to send
out updates at the proper times.

Signed-off-by: Pierre Ossman <ossman@cendio.se>
Message-id: 5aa28297d665cee24ddab26bbf4633e4252f97b6.1483978442.git.ossman@cendio.se
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>