Added support for gst_flow_event_latency and gst_flow_event_qos LTTng tracepoints.

The gst_flow_event_latency tracepoints output the latency field of the GST_EVENT_LATENCY event.
This tracepoint immediately follows the gst_flow_event tracepoint and precedes
the gst_flow tracepoint on the same thread.

The gst_flow_event_qos tracepoints output the proportion and diff fields of the GST_EVENT_QOS event.
This tracepoint immediately follows the gst_flow_event tracepoint and precedes
the gst_flow tracepoint on the same thread.

Signed-off-by: Alexander Barabash <alexander_barabash@mentor.com>