Fixes to VobSub subtitles loading

The first commit fixes manual loading of VobSub subtitles, while the second, larger commit moves the matching of .idx/.sub files to happen earlier in the autoloading process to avoid several pitfalls. The commit log entries contain more detailed information.

Comments welcome, especially regarding the second commit and the SubFile and SubFileList there in Util.h (not sure if it is preferred to do that in some other way).