tmf: Allow time graph zoomed event list to be built incrementally

tmf: Allow time graph zoomed event list to be built incrementally

Instead of being replaced with a whole new list in one operation, time
events can now be incrementally merged into the zoomed event list.

The time graph entry adds a method to add time events to the existing
zoomed event list, replacing the last event if necessary.

NullTimeEvent is updated to correctly split without losing its type.

Change-Id: Icf51199483244a61580592d3f193ef460eded655
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/52964
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>