Add setting to blur and unblur subtitle tracks on mouse hover

[nekorushi]

Added setting that allows for blurring of selected subtitle tracks. When mouse hovered, they unblur.

[killergerbah]

@nekorushi Thanks for taking the time to contribute your idea and implementation. The only major feedback I have is that I would prefer if the data model for the settings future-proofed itself against the possibility of more track-specific settings. This would make it more trivial to implement more of these types of settings for more than 3 tracks, with less code duplication, and with less hardcoding of keys that look like foo1, foo2, foo3, .... For example, I have received a feature request to make subtitle alignment track-specific as well, and the implementation for this would be simplified if the data model already readily supported track-specific settings. This is my suggestion for the settings data model:

subtitleTracks: [{"blur": true/false}, {"blur": true/false}, ...]

[killergerbah]

Nitpick: Hides selected subtitle tracks by blurring them. Can be un-blurred on mouse hover.

[killergerbah]

Thanks @nekorushi for your contribution. It is close to merge, can you just make yarn run verify pass? You can update the loc keys for non-English languages by running node merge-loc-from-en.js inside of scripts.