HeuristicLab graph layout and visualization

[bburlacu]

This would provide functionality for the layout and visualization of graphs represented using the DirectedGraph in HeuristicLab.

The functionality should be split between two plugins:

• A graph visualization plugin providing a DirectedGraphView and a wrapper class for the layout algorithm (provided by an external library)
• An external library plugin for the adaptagrams library used for layouting and edge routing. The external library (​http://www.adaptagrams.org/) is compiled from source code (LGPL) for i686 and x86_64 targets and invoked via an automatically-generated C# wrapper (created with SWIG). The wrapper plugin is responsible for selecting the correct library version according to the build target (32 or 64bit).

The two plugins described above depend directly on the HeuristicLab.Visualization branch (#1265), therefore should also live in the same location.