- added /sim/logging/classes and /sim/logging/priority properties

- added /sim/logging/classes and /sim/logging/priority properties
  to control logging
- /sim/logging/classes takes a value like "terrain|astro", where the
  names are the macros from simgear/debug/debug_types.h with the
  initial "SG_" removed and converted to lower case; for none, use
  "none"
- /sim/logging/priority takes a single name like "warn", where the
  name is a macro from simgear/debug/debug_types.h with the initial
  "SG_" removed and converted to lower case