Line: Side relative accessors; made private more instance data

Line::Side now features a set of side relative accessors enabling
users of the class to implement their algorithms more expressively.

Made use of the new functionality to simplify various algorithms
in the FakeRadio and light decoration modules.

The client-only FakeRadio data for line was moved to that module.

Applied the pimpl idiom to Line::Side and Line::Side::Section and
added a Line::FlagsChange audience (which Line::Side observes on
client side).

Plus significant clean up.