Required (minor) changes for GuiSvgPlot Integration

For the integration of brailleplot into GuiSvgPlot it is necessary to change some visibilities so that the BraillePlotService created for GuiSvgPlot can use the functionality (e.g. building user defined formats in code).

Other additions:

• central Diagram class that holds title and captions/units for x-axis and y-axis. GuiSvgPlot builds such a Diagram object and passes it to the MasterRenderer as Renderable
• BarChartRasterizer not crashing when receiving empty diagram data (Yes, GuiSvgPlot does this sometimes.)