tmf: Add a trace-getting method in TmfView

tmf: Add a trace-getting method in TmfView

Instead of calling the getSite.getPage.getEditor mantra every
time, if anyone has a reference to a view they can now use this
convenience getActiveTrace() method to get the currently selected
trace.

Kind of replacement for the old TmfExperiment.getCurrentExperiment();

Change-Id: I84783c6e65bb5c9095d4f868e9cb00a80bb246b0
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/11530