tmf: Add support for time graph content provider

tmf: Add support for time graph content provider

The time graph viewer now supports a time graph content provider. This
allows for the input to be something else than an array of
ITimeGraphEntry elements. The refresh method can now detect new or
removed root elements freshly derived from the input by the time graph
content provider.

Change-Id: I4aeaf670d5d459a5a176a6e12645932c13c59413
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/21160
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>