Restore external ass support (again)

@valib: It's happened again it seems :-).

I started to pepper in a lot of ```getType() == SubtitleType.ASS``` conditionals but it began to look silly, especially since ```getSubtitles()``` code now looks to be in a transitional state with a lot of duplication.  Anyhow I took the liberty of trying to simplify it in a (hopefully) beneficial way and I'd appreciate if you test and correct any flaws from your perspective. Also please note the second commit, which attempts to rely less on the ```getSubtitlesCodepage()``` user setting.