[settings] reshuffle, group and add conditions for enabled/visible status for better usability of settings screens
[language] fix wrong and misleading labels
[settings] cleanup video decoding settings (hw-accel + frame-multi-threaded) by moving them to their own settings category
[subtitles] change settings logic for subtitle storage location (make it a spinner)
[settings] enable teletext scaling only if teletext is enabled
[settings] pvrpowermanagement settings/timers to be dependent on powermanagement being enabled
[subtitles] drop 'prefer external' option, always preferring
[settings] drop unused videolibrary.seasonthumbs
[settings] move 'flatten library' into video->library