ENH: Convert WindowLevelHelper to VoiLutHelper and make it support VoiLuts.
ENH: Add support for VoiLut to ImagePipeline.
ENH: Add support for VoiLut to VolumeDisplayUnit.
ENH: Add initial support for VoiLut to Q2DViewer.
ENH: Rename setWindowLevelPreset() to setVoiLut(). This should be the main (and maybe the only) method to set any kind of VOI LUT (either window level or LUT) to a QViewer in the future.