Implement LV2 files extension.

Implement LV2 files extension.

This extension works with the persist extension to allow plugins to
save references to files in their state, but still allow the host
(e.g. Ardour) to control where files are located, making it possible
to do a deep export of a session with all files (e.g. loaded sampler
banks) included.

Tested working with a patched version of LinuxSampler...

TODO: Make symlinks in the plugin state directory rather than refer
to absolute file paths in state files (i.e. support session export,
either by ardour or any other tool, in particular the Jack session
manager...)

git-svn-id: svn://localhost/ardour2/branches/3.0@9272 d708f5d6-7413-0410-9779-e7cbd77b26cf