TMF: Add the possibility to specify a content provider for time graphs

TMF: Add the possibility to specify a content provider for time graphs

This gives more flexibility to manage time graph entries instead of
building the list only once when the trace is loaded.

For example, a view like the Critical Path View where the list of
entries depend on the selection from another view can be managed using a
content provider.

Change-Id: I5de7b5c16ba3b60c26f7d0d4a69289f6ddb8441b
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/41476
Reviewed-by: Hudson CI