tmf.ui: fix callsite and model lookup regression

tmf.ui: fix callsite and model lookup regression

The notion of ITmfCallsite is lost when an event is cached as it is now
an CachedEvent which does not implement ITmfCallsite. This patch checks
if the event is cached, and if so, resolves the parent event to
validate.

Change-Id: I99733958bd1ac797fca32c8b293e709f4ddc3cc5
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Signed-off-by: Bernd Hufmann <Bernd.Hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/37120
Tested-by: Hudson CI