callstack: Move CallStack* types to core.callstackAs these shouldn't stay in the core (plugin root) java package. Movethem all few into a new core.callstack sub-package.Change-Id: Ide304906df7f288d12c3f5c9dc52e3476b3bca61Signed-off-by: Marco Miller <marco.miller@ericsson.com>Reviewed-on: https://git.eclipse.org/r/c/tracecompass/org.eclipse.tracecompass/+/201332Tested-by: Trace Compass Bot <tracecompass-bot@eclipse.org>Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>