Python-bindings: Refactor the Context class

The context class is now called TraceCollection and provides generator
functions to iterate over the traces' events. This makes it possible to
get rid of the Iterator, IterPos and File classes.

The examples are adapted to match the changes.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Python-bindings: Refactor the TraceHandle class to use properties

TraceHandle now exposes properties instead of getters. It also keeps
a reference to its parent TraceCollection.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>