ui: deprecate use of SDL 1.2 in favour of 2.0 series

The SDL 2.0 release was made in Aug, 2013:

  https://www.libsdl.org/release/



That will soon be 4 + 1/2 years ago, which is enough time to consider
the 2.0 series widely supported.

Thus we deprecate the SDL 1.2 support, which will allow us to delete it
in the last release of 2018. By this time, SDL 2.0 will be more than 5
years old.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180115142533.24585-1-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>