Use new Lilv state API to save LV2 plugin state.

Use new Lilv state API to save LV2 plugin state.

This saves a complete history of plugin state, i.e. save is no longer destructive.  However, data is shared as much as possible, and new state is only written if the plugin state has actually changed.  There is exactly one link in the entire session directory to any external file, so archiving will work with minimal copying.

Not sure sure about the naming of the "externals" directory, but I have nothing better...

git-svn-id: svn://localhost/ardour2/branches/3.0@11372 d708f5d6-7413-0410-9779-e7cbd77b26cf