Add VLSub feature: Download multiple subtitles

All selected subtitles are now downloaded, instead of only one.

Change 'get_first_sel' function to return ALL selected subtitles
instead of only one, and refactored name to 'get_all_sel'.

Change 'download_subtitles' function to iterate over 'get_all_sel'
returned table instead of downloading the only subtitle from old
'get_first_sel'.

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>