Tmf: Move some TmfTraceElement methods to parent class

Tmf: Move some TmfTraceElement methods to parent class

Experiment elements and trace elements now behave more alike, as a first
step to have full support of experiment types.

Change-Id: Iad5e2c973300f6f1ce155513e10ce1b390231223
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/20668
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>