fix #428919: Support multi-trace histograms

fix #428919: Support multi-trace histograms

When displaying a multi-trace experiment, colorize the histograms,
in order to know the origin of the events.

Change-Id: I9436a4b8d08250e41b7e68a46757b85a9797d658
Signed-off-by: Xavier Raynaud <xavier.raynaud@kalray.eu>
Reviewed-on: https://git.eclipse.org/r/22442
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>