For making an output secure, the content-protection should be set for
each of head attached to that output. So whenever the protection for
a weston_output is desired, it means that protection is desired for
each of the weston_head attached to that weston_output.

This patch introduces a new enum in libweston to represent the
requested/current protection statuses, equivalent to the type enum
defined by the weston-secure-output protocol. The new enum helps to
extend the content-protection status and requests to libweston and
the backends.
This patch also adds a new member desired_protection to store the
desired protection for an output in weston_output.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>