[LV2] New discovery API (Was: lv2_descriptor() considered harmful)

[David Robillard]

I added the discussed new discovery API to lv2.h and implemented it in
lilv and Ingen svn.

If anyone else who mentioned needing this could implement it and provide
some feedback/testing that it works with another implementation (e.g.
Lilv) please do so.  A release is well overdue but I can't release this
until it's actually implemented somewhere.

-dr