Organise libbabeltrace

Move the library from converter/ to lib/.

Split functionnalities into separate files :
- babeltrace.c
- iterator.c
- callbacks.c

New internal include files for shared structures between iterator and
callbacks :
- include/babeltrace/callbacks-internal.h
- include/babeltrace/iterator-internal.h

No new functionnalities, just a cleanup of the structure.

Signed-off-by: Julien Desfossez <julien.desfossez@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>