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" or "visible" dependencies to settings where they where missing IMO to either grey them out or hide them entirely if not needed to have less visual clutter and confusion (i.e. ReplayGain and Crossfade settings).
I also began to merge splitted up strings of multiline dialogs into one string and use the "textbox dialog" instead of the multiline one. But I heard this might not be wanted at this point in release cycle, which is why I made it a separate commit and stopped with it for now, waiting for more opinions. Can drop it for Gotham.

Settings handling is still not consistent all over the place. Some sections like "karaoke" have all subsettings disabled (but not hidden) if main feature is disabled while this is not the case for PVR (everything is shown all the time, not even disabled). Also I'd like to hide "colors" and "themes" from the skin settings if the skin doesn't support them, but haven't found out how to. Feel free to give more suggestions for cleanups and usability improvements on our settings pages which I should add, it's just a start.

Next step for me would be to write missing settings descriptions - volunteers to team up are welcome @Martijn @jjd-uk :)