This is a clean up commit prior to the subsystem API standardisation to simplify
the diffs. It includes all SGSubsystem and SGSubsystemGroup derived classes.
fgpanel is basically the stripped down 2D-Panel code from
FlightGear. It is designed as a standalone lightweight panel
rendering engine to draw 2d panels on a lowcost computer/graphic card
without 3d acceleration at reasonablel framerates.
Patches for inclusion into the build system will follow.