A few tweaks on Feature/CustomSubs

- Add subs with File Dialog.
Works good, but we still need css classes for the button.

- Support adding multiple files at the same time.
Both Drag&Drop and File Dialog

- Removed e.preventDefault and stopPropagation on 'drop' event:
preventDefault was redundant, since we already prevent defaults on app startup (https://github.com/Cuevana/storm/pull/8)
stopPropagation shouldn't have been there in the first place.. because we might want, in the future, catch these kind of events from different places.


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Cuando la app sea traducida, y la audiencia objetivo abarque varios idiomas, va a ser necesario detectar el charset de los subtitulos.. sino se van a ver mal.