tmf.ui: Use a job for the time graph view's build thread

tmf.ui: Use a job for the time graph view's build thread

This allows swtbot's waitForJobs to wait for the build thread to complete.
It also prevents from having to instanciate a monitor.

Change-Id: Idbb5031b0343ba44e39ff6a1ba0f06b2e764a6de
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/75279
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-by: Hudson CI