Fix LV2 preset deletion and clash between plugins.

Fix LV2 preset deletion and clash between plugins.

Before this, LV2 preset deletion in Ardour was doubly broken: the wrong file
was being removed, and removing the correct file would only result in a broken
preset.  This change uses a new version of Lilv which has a more sophisticated
mechanism for preset deletion.

Also, fix "clashing" presets saved with the same name for different plugins, by
prefixing the plugin name to the bundle (this is now a recommendation in the
LV2 preset specification).