tmf: Support child entries with different state systems in TimeGraphView

tmf: Support child entries with different state systems in TimeGraphView

Decouple the roots of the AbstractTimeGraphView with the entries
associated with a state system in AbstractStateSystemTimeGraphView.

This allows, for example, a 'trace' entry to be root of the graph, that
has independent branches of children whose event lists, links and
markers are populated using their own, separate, state systems.

Change-Id: Ied2a2f91d6661e4cff76f07c06ba9e2eb5c4e25c
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/95730
Reviewed-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>
Tested-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>
Reviewed-by: Hudson CI