gtkmm-plplot-2.2

• RegionSelection: add scroll zooming
• RegionSelection: add support for selection box color and width
• RegionSelection: add panning support
• PlotDataLine: new class, acts as parent for Plot2D and PlotDataHistogram
• PlotDataHistogram: new abstract class, parent for PlotDataHistogramUnbinned and PlotDataHistogramBinned
• PlotHistogram: new class for a plot containing a single PlotDataHistogram dataset