Feature: Add Custom Subtitles from Local Drive

Add local subtitles (*.srt) by dragging them into the player. (https://github.com/Cuevana/storm/issues/1)

Encoding is/will be an issue, I assume ISO-8859-1 (Spanish) -- as I seen in the rest of the app. But won't work properly with other languages. We'll need a charset detector, or maybe guess based on language.

TODO: UI Design. I added a "tip" on the subtitles menu. I think it should be styled to be smaller or maybe completely removed.