Play -> Subtitles menu: Improve the external subtitles naming.

This pull request is just here to discuss the different naming choices.

**Current way (bad)**
* if subtitle filename is of the form `<video.filename><something>.<ext>` it will show `<something>`
* else it would show `Undetermined`.

**Proposed commit**
* if `<video.filename>.<ext>` --> `<ext>`
* if `<video.filename><something>.<ext>` --> `<ext> [<something>]` with `<something>` converted to a language name whenever possible (NB: probably a separator between `<video.filename>` and `<something>` should be mandatory, good question is what an acceptable separator is).
* else full subtitle filename would be shown.

**Other option: always show the filename with inner ellipsis when too long**
* it would be still possible to parse the filename to show `(type) (lang)` at the end so those info are obvious and the naming similar to LAV's.
* if the filename is  `<video.filename><sep><something>.<ext>` we could replace the filename by just the `<something>`.