TMF: Move methods from TmfXYChartViewer to new abstract TmfTimeViewer

TMF: Move methods from TmfXYChartViewer to new abstract TmfTimeViewer

The TmfXYChartViewer already had time synchronization methods implemented,
but those can be used by other types of viewer. They have been moved to an
abstract class. All viewers who need to synchronize with trace time may extend
this class.

Change-Id: I5d64c4412ea2de06e88ae8000b29770dbeecca83
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/22029
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>