scanner: Add version argument to wayland-scanner

This adds a command line argument to print wayland-scanner version.

It also makes wayland-scanner emit a comment with wayland library
version to every file it generates.

v2: separate variable definitions into their own lines and remove
    old style "version" argument

Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Tested-by: Yong Bakos <ybakos@humanoriented.com>
Signed-off-by: Armin Krezović <krezovic.armin@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>