tmf: Bug 494767: Events in selection not updated in Statistics view

tmf: Bug 494767: Events in selection not updated in Statistics view

- Make sure the time range created from TmfSelectionRangeUpdatedSignal
always has the earliest time as start time.

- If the events per type map is empty for a time range stats update,
reset all time range event counts.

Change-Id: I2c860bdd0dd9e033933c690f230aaf79e6f60b5e
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/73973
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>