Rework content panel selection to be more generic

This removes also some hidden and still unused entries for future content panels.

Signed-off-by: Simon Busch <morphis@gravedo.de>
DevOptions: don't use white font color

Signed-off-by: Simon Busch <morphis@gravedo.de>
SystemUpdates: cleanup layout a bit and remove superfluous things

Signed-off-by: Simon Busch <morphis@gravedo.de>