libweston: create/find output by name

To let users pick an arbitrary name for an output, to be used as e.g. a
configuration key, add API to create an output with a given name. For
the same configuration purpose, add a search function as well.

For the search function to be predictable, forbid creating multiple
outputs with the same name. Previously, creating multiple outputs with
the same name would have needed detatching to create outputs from the
same head, now that is forbidden.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>