tonecurve: display usual guides whatever histogram mode is chosen

The tonecurve module displays the histogram as its widget background
and currently changes the way guide lines are drawn depending on
histogram widget mode.

However the tonecurve module doesn't work in any new fancy histogram
way, so no use to take care about new guide lines for these modes.
Merge pull request #378 from edgomez/bugfix

tonecurve: display usual guides whatever histogram mode is chosen