[LV2] lv2:relation

[David Robillard]

Since someone is urgently waiting on this functionality, I have taken
the liberty of abusing my authority and committing :)

http://lv2plug.in/trac/changeset/565

Lilv SVN allows access to this information with:

/**
   Get a port on @c plugin by the lv2:relation it represents.
   If the plugin has multiple ports with the same relation (which it
   should not), the one with the lowest index will be returned.
*/
LILV_API
LilvPort*
lilv_plugin_get_port_by_relation(const LilvPlugin* plugin,
                                 const LilvNode*   relation);

This has me realizing we should have just described control ports as
properties from the get-go.  Oh well.

-dr