TMF: Add vertical events (links) to time graph view

TMF: Add vertical events (links) to time graph view

Add a new interface ILinkEvent and class TimeLinkEvent, for events that occur
between two entries instead of in one entry only.
Add the getLinkList method to AbstractTimeGraphView to get the links for a
given zoom level and other methods to propagate those changes to the time graph
control

Change-Id: I5be30c4a27a10645a57ebfc13b4d1c71a55e06bd
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/15105
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>