[vobsub] always set correct source value when creating vobsub stream in demuxer
[vobsub] set filename2 on all vobsub streams that are part of the external file
[vobsub] don't prioritize external VOBSUBs over external SRTs
[vobsub] ensure external vobsubs included in the idx file are all marked as (external)
[vobsub] exclude archived .sub files from filename when not starting with base video name
Merge pull request #5414 from Voyager1/vobsub-fixes

VobSub fixes