plugins: don't lock wavetables

while wavetables could be generated within synths, this use-case is very
unlikely. therefore we relax the locking requirements for wavetables.

affects supernova only

Signed-off-by: Tim Blechmann <tim@klingt.org>
plugin interface: add doxygen documentation for SCUnit class

Signed-off-by: Tim Blechmann <tim@klingt.org>