tmf: Add a "Trace" Event Table column for experiments

tmf: Add a "Trace" Event Table column for experiments

Also reworked some internal methods in TmfEventsEditor to not return
a table object directly, but rather just a set of event aspects.
This way we can add this "Trace" column only for experiments.

Change-Id: I21536cd11da28f30b1f822a495cd06a4d922064c
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/37122
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>