Better Handling of Node image files

[dotasek]

Currently, the format of the VisualProperty for setting images is cryptic and poorly documented. Example of a valid value:

{
    "visualProperty": "NODE_CUSTOMGRAPHICS_1",
    "value": "org.cytoscape.ding.customgraphics.bitmap.URLImageCustomGraphics,18,bundle://76.0:0/images/sampleCustomGraphics/Arabidopsis_thaliana_L.png,bitmap image"
  }