weston: migrate headless to head-based output API

Migrate the headless frontend to use the new head-based output
configuration API: listen for heads_changed, and process all heads.

The simple_heads_changed() function is written to be able to cater for
all backends. The rest will be migrated individually.

The head destroy listeners are not exactly necessary, for headless
anyway, but this is an example excercising the API. Also
is_device_changed() check is mostly useful with DRM.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>