linux.ui: Add context menu to Control Flow View to follow a thread

linux.ui: Add context menu to Control Flow View to follow a thread

The command to follow a thread generates a TmfThreadSelectedSignal. To
see the context menu, one must right click on a thread entry in the control
flow view. More follow action locations can be:

 * the events table
 * the resources view, on an event
 * the filters view
 * the critical flow view

Change-Id: Ib74eb67f98c514f29fde689c47c2cb42c90868db
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/66665
Reviewed-by: Hudson CI
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>