tmf: Show closest event tool tip when hovering over blank space

tmf: Show closest event tool tip when hovering over blank space

Show the tool tip for the closest between the next and previous event
when hovering over the blank space of a time graph entry, if the
distance to the event position is within a threshold.

Change-Id: I8a95d488e03c1d5012a1ee0bd4597f863b2ba3b6
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/65612
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>