tmf: Expose TmfExperiment.getTraces() up to ITmfTrace

tmf: Expose TmfExperiment.getTraces() up to ITmfTrace

When a view should display information about either 1 stand-alone
trace or an experiment, it will be able to call trace.getTraces()
directly, without having to do clunky instanceof checks every
single time.

Change-Id: I2f962a1dc4e6e34bf956aa0e62ba8b42cb234a99
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/11531
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bhufmann@gmail.com>
Tested-by: Bernd Hufmann <bhufmann@gmail.com>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>