Add reference counts to state and stats saved attributes. This way, the

information can be shared among several contexts using the same trace but
is freed when all contexts are "finished". Summing statistics is now
done separately to permit more reuse for trace statistics (same trace
in several tracesets). The statistics viewer does not yet call this.

Use a more generic lttv_attribute_write_xml xml writer to save stats.

Place the .h files along with their base package instead of in a
separate include tree.

git-svn-id: http://ltt.polymtl.ca/svn@500 04897980-b3bd-0310-b5e0-8ef037075253