tmf: Specify extended trace properties with an interface

Introduce a new ITmfTraceProperties that traces can implement when
they can provide additional trace properties. Also make the the
Enviornment Variables view use this interface, so it is not specific
to CTF traces.

Change-Id: I9a6901f720bc028f28ce5c180b778701a8ec0f5b
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/12789
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>