tmf: Add a way to clear the traces using SWTBot only

tmf: Add a way to clear the traces using SWTBot only

There is way to clear traces but it deletes trace elements
directly using TMF APIs. It would be good to have a way to
do it completely in the UI using SWT. This patch updates
ProjectExplorerTraceActionsTest to use the new way so that
it tests the UI more fully.

Change-Id: If0a8908ecb3f8b0c209ce3c9c8cca883e941be5d
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/79793
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>