Added trace attributes to properties:

Added trace attributes to properties:

TRACE_READ - log a message whenever the property is read.

TRACE_WRITE - log a message whenever the property is written.

The second one works only when the property value is changed through
the property manager; tied variables and accessors are not polled for
value changes because of the performance hit.

These methods end up invoking private methods
SGPropertyNode::trace_read and SGPropertyNode::trace_write.  By
setting breakpoints on these methods inside a debugger, it is possible
to debug property access and find what parts of a program are reading
or writing specific property values by doing a backtrace.

In the XML property files, users can use the attributes 'trace-read'
and 'trace-write' to control tracing; the value should be 'y' to
enable tracing or 'n' to disable it (the default).