[RFC] improve subtitle settings and their handling

This is not meant to be merged yet as it's entirely untested. It's for now just a RFC to get your opinions if those changes would work for everyone.

Changes done:
a) move "subtitles.languages" to a more generic "locale.spokenlanguages" setting
My main intention to do this is to have more generic setting to store the languages the user speaks or understands that can be used for other things as well. I plan to add a setting to limit subtitles and audio tracks to spoken languages only, which comes in quite handy if you i.e. watch blurays with 8 audio tracks and 18 subtitle tracks and you cycle through them.

b) change subtitle storage configuration
I didn't like the "Save subtitles to movie folder" checkbox together with the "Custom subtitle folder" as they where not grouped and it was not really obvious how those two interact. Now I added a "subtitle storage location" spinner with two options: "use movie folder" and "custom folder". And once custom folder is chosen, the according setting will be enabled.

c) rearranging settings
well, to me it was way to cluttered and settings where not in logical groups and some IMO had wrong labels


A next step would be to move the "prefered subtitle language" from "Appearance -> Localization" to "Video -> Subtitles" and do the same for "prefered audio language" ("Video -> Playback")