python: reimplement the babeltrace package as a bt2 wrapper

python: reimplement the babeltrace package as a bt2 wrapper

This is how the old bindings work with the new bindings:

* A `babeltrace.TraceCollection` contains a set of
  `babeltrace.TraceHandle`, each of which only contains a CTF trace path
  and a generated ID, unique within the trace collection.

* When you call `babeltrace.TraceCollection.add_trace()`, the function
  makes sure that you're adding a single CTF trace. It creates a trace
  handle, adds it to the trace collection's set, and returns it.

* When you call `babeltrace.TraceCollection.remove_trace()`, the
  function removes the given trace handle from the trace collection's
  set.

* The `timestamp_begin()`, `timestamp_end()`, and `_has_intersection()`
  methods of `babeltrace.TraceHandle` perform the `trace-info` query
  with their path to retrieve the information.

* The `babeltrace.TraceHandle.events` property creates a `bt2` trace
  collection notification iterator, gets the first notification of the
  trace (which is expected to be a stream beginning notification), and
  finds the CTF IR trace object to generate all the event declarations
  from event classes using
  `babeltrace.reader_event_declaration._create_event_declaration()`.

* The `babeltrace.TraceCollection.events` property creates a `bt2` trace
  collection notification iterator, in stream intersection mode if
  needed, and with a beginning and end time if needed, and generates
  event objects using `babeltrace.reader_event._create_event()`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>