README: add note about versioning
configure.ac: check for wayland version

if it's lower than wit's version display warrning
core: add configuration

add structure which represents the configuration.
It can be used to allow/suppres creation of any object in the diplay
(except for the ones hardcoded in wayland i. e. wl_display)
client: populate header

make functions private
tests: add tests for user func and configuration
server: create globals on start

create globals according to configuration (now only seat)
Also add default seat implementation (requests)
wit_config: move wit_default_config into server.c

because of redefinition in wit-server-protocol.c