tmf: Make ITmfGraphEntry.getChildren() return a List

tmf: Make ITmfGraphEntry.getChildren() return a List

instead of a static array.

Because of the upcoming AbstractTimeGraphEntry and its subclasses,
we will now need to specify the exact class in each view's
.getChildren(). This is not possible to do with an array if we use
a generic type (e.g. E[]), we have to use a Collection like List<E>.

Change-Id: Icf858b471a95ea7d68e6b6b2169afdc35922545a
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/7747
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>