Label and settings UX cleanups

I took the time to go thorugh all settings and check them for usability issues. Doing so I found some wrong, inconsistent and/or misleading labels, i.e. in PVR settings and subtitles. Also the arrangement and visibility of certain settings didn't make sense to me, which is why I shuffled things around (like the "prefered audio/subtitle language" settings) and defined "parents" for settings which I though are apropriate to visualize the dependencies for better understanding (indention). Additionally I added some "enable" dependencies to settings where they where missing (i.e. ReplayGain and Crossfade settings).