Extend properties to support new property types.

Extend properties to support new property types.

An SGRawBase class has been added as a base class to the SGRawValue
hierarchy so that SGPropertyValue functions don't necessarily need to
know the type of the value stored in the node.

A new SGRawValueContainer class stores properties that shouldn't be
stored in the node itself. PropertyTraits indicates if a type is
stored in the property node or externally.

Add getValue and SetValue template member functions to SGPropertyNode.

Read and write new extended properties.

Rearrange props.hxx a bit so that the template magic actually works.

Split out extended raw value virtual functions into a seperate base class.

SGRawExtended is chosen as a base class of SGRawValue for extended property
types.