weston-debug: Introduce weston_log_subscription and weston_log_subscriber objects

Adds a minimalistic API for managing the subscription object. The
subscribe functionality will be brought in once we re-organize a bit
weston-debug-stream and split it properly. It extends the logging
context with a linked list of potential subscription and adds a linked
list of subscriptions in the log scope.

This patch represents the start of a logging framework for weston. It's
being built around weston-debug, with the intent to superseded it, and
make weston-debug a client of the framework. Further more the logging
framework should replace current logging handler and allow other types
of streams to be used.

Currently present in libweston under weston-debug we have log scopes, debug
streams and a logging context.

With this patch, two (internal) objects are being added: the concept of
a subscriber and the concept of subscription.  The subscription object
is a ephemeral object, implicitly managed which is created each time one
would want to a subscribe to a scope. The scope will maintain a list of
subscriptions and will continue to be explicitly managed.

The streams will use the subscriber object as a base class to extend
upon. By doing so it allows to customize the stream with specific
functions that manipulate the underlaying storage. The subscriber object
will require a subscribe function and specific stream functions and like
the scope, will be explicitly managed.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>