compositor: introduce structs to handle backends configuration
compositor: introduce structs to handle backends configuration

This commit introduces the structs weston_backend_config and
weston_backend_output_config, to prepare for the new config
system for the backends.

Reviewed-by: Bryce Harrington <bdb14cfc1ab8d24e13de84adc40b41ae60c13bd1@osg.samsung.com>

kwm81/weston,sir-murray/weston,Gnurou/weston,giucam/weston,krezovic/weston,sir-murray/weston,kwm81/weston,Fantu/compositor-spice,Gnurou/weston,Fantu/compositor-spice,krezovic/weston,Fantu/compositor-spice,giucam/weston