gst-stats: Add element latency support

gst-stats: Add element latency support

This will output latency information when parsing a log file with gst-stats that
has latency trace information. It will show the min, max and mean latency for
the pipeline and all its elements. It will also show the reported latency for
each element of the pipeline. Output example:

Latency Statistics:
	pulsesrc0_src|fakesink0_sink: mean=190000043 min=190000043 max=190000043

Element Latency Statistics:
	flacparse0_src: mean=45561281 min=654988 max=90467575
	flacenc0_src: mean=89938883 min=81913512 max=97964254
	flacdec0_src: mean=45804881 min=228962 max=91380801

Element Reported Latency:
	pulsesrc0: min=10000000 max=200000000 ts=0:00:00.262846528
	flacenc0: min=104489795 max=104489795 ts=0:00:00.262898616
	flacparse0: min=0 max=0 ts=0:00:00.262927962