libweston/weston-log: Add a iterator helper for debug scope

This adds three new helpers: one to iterate over all debug scopes
created/added and other two are for simpler getters for the scope name
and the description.

Included with this change is also a simple test to retrieve them.

This is an alternative to using the debug scope list advertised when
using the weston-debug private extension. libweston users can use this
directly to know which scopes they can subscribe to, and there's no need
to have a client implementation for the weston-debug protocol.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>