introduce new headers wayland-client-core.h and wayland-server-core.h

wayland-client.h and wayland-server.h include the protocol headers generated
at build time. This means that a libwayland user cannot generate and use
protocol code created from a wayland.xml newer than the installed libwayland,
because it is not possible to only include the API header.

Another use case is language bindings, which would generate their own protocol
code and which only need to use the library ABI, not the generated C code.

This commit adds wayland-client-core.h and wayland-server-core.h which do not
include the protocol headers or any deprecated code.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>