Bump the future release to 3.0.0 due to breaking ABI in libweston.

We have merged a few patches already that change libweston/compositor.h.
While most of the changes arguably change only things libweston users
should not be touching, some change the size of e.g. struct
weston_output and struct weston_compositor, possibly moving member
offsets. We also haven't separated public and private parts from
compositor.h yet. To be on the safe side, bump the major now. I'm sure
there will be more changes that make the bump obviously necessary.

Cc: Bryce Harrington <bryce@osg.samsung.com>
Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emilio Pozuelo Monfort <emilio.pozuelo@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>