tmf: Update time graph control drawing order

tmf: Update time graph control drawing order

Draw the item's name and middle line as part of the background layer.
This allows grid lines, and later markers, to be drawn on top of the
middle line.

Change-Id: Icfe9fcdaa7c39bfe2bc9c055929a86f157b21773
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/56372
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>