tmf: Use the trace manager instead of ITmfTrace.getTraces()

tmf: Use the trace manager instead of ITmfTrace.getTraces()

Planned revert of 18991e1, now views can call the trace manager
to get the list of active traces, no matter if it's inside an
experiment or not.

Change-Id: I592de26a5edb4a8b8ed82a4442cb2b59d488f0f1
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/12424
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>