Added the functionality to change the icon style of all vertices at once. It is accessible via a new sub-menu when right-clicking outside an edge / vertex. (Closes #30).
Added the option to disable / enable node icons. The old CustomVertexRenderer is used for drawing the graph without the icons.
The preference about enabling / disabling the vertex icons can now be saved and loaded from the .graph file. Added a custom PajekNetwWriter class and moved it to a separate package along with the CustomPajekNetReader. Also the options to change the node icons are no longer available if the icons are disabled.