[rbp] Disable unimplemented settings options

I've been through the settings carefully and have made any settings that are not implemented on Pi be not-visible.

If and when support is added for probing capabilities of the media player then the hiding of unsupported options can be removed.

Thanks to montellese for making this configuration so simple.
Merge pull request #2774 from popcornmix/rpi-settings

[rbp] Fix new settings scheme for RPi and provide defaults