add dvdnav_toggle_spu_stream

Another one that I was not able to accomplish using the dvdnav interface only. This PR adds dvdnav_toggle_spu_stream which allows the client application to enable or disable the current subtitle stream (or in other works - to toggle its visibility). Allows to remove the same functionality we had in kodi for ages (accessed via the VM directly). Runtime tested replacing the snippet with:

m_dll.dvdnav_toggle_spu_stream(m_dvdnav, static_cast<uint8_t>(bEnable));

Cheers