This patch adds support for directional links in libimagentrylink.
A directional link is added to the same header as the "internal" links,
which are unidirectional.
The "outgoing" link is added in "links.to", the "incoming" links are
added to "links.from".
This way we can always traverse all linkage. The `Linkable::links()`
method is changed to always return _all_ link.
The `Link` type was extended for notions of "to" and "from" links.
Also adds testing.
Signed-off-by: Matthias Beyer <mail@beyermatthias.de>
Matthias Beyer
0eac6d9931