tmf: Make TimeGraphEntry implementation less restrictive

tmf: Make TimeGraphEntry implementation less restrictive

A TimeGraphEntry's parent and children no longer have to be instances
of TimeGraphEntry. They can now be any type of ITimeGraphEntry.

The method addChild(int, ITimeGraphEntry) is added to insert a child at
any position.

Change-Id: I8144c7f06505c3a2a4c56264e1543fab84a77be5
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/29829
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>