bt_trace_class_create(): accept mandatory self component

This patch makes bt_trace_class_create() accept a mandatory self
component. The self component parameter is not used now, but it could be
in the future to control allocation (object pooling) and for
bookkeeping, as well as to have a system where a new trace class within
the graph can lead to notifying other components about it so they can
prepare associated data.

The `ctf` plugin is changed so that it does not require a trace class to
create packet indexes. This is required because the `trace-info` query
requires packet indexes to exist, but there's no self component in this
context to create a trace class.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>