add dvdnav_set_active_stream

Provided the physical stream index and its type (audio or subtitles) set it as active on the vm.

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Ok so... last one I miss from the dvdnav interface to finally get rid of the private VM usage in Kodi (hoping the other 2 pending PRs also go in). A way to activate a given subtitle or audio stream provided its physical index:

• The provided index is validated before usage.
• Reuses the same dvdnav_stream_type_t recently introduced
• The indexes are consistent with the other functions available on the lib interface (e.g. dvdnav_get_spu_attr(...,uint8_t audio_num) or dvdnav_get_spu_logical_stream(..., uint8_t subp_num)

Just for reference, the Kodi implementation this tries to avoid is available here.

Runtime tested with: https://github.com/enen92/xbmc/commit/5be53259936e150f49d34cbf59ee5dd5b5ab31b1

The PR will need to be rebased when (and if) !36 (merged) is merged as it adds code to the same subsection.

Cheers