tmf: Fix histogram label background to be theme-friendly

tmf: Fix histogram label background to be theme-friendly

The histogram text controls are now labels and no longer have their
background color set so that they can naturally inherit from their
parent's background whenever the color changes due to active/inactive
view part according to the theme.

The histogram filler area now takes its color from the parent
composite's current background color whenever it is redrawn.

Change-Id: I81ea92863186749dac4e11002b29899d80c4b97a
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/27214
Tested-by: Hudson CI