General and visual support of node shapes

Node shapes is an important element in network visualization design and results. Previously some tests has been made to support image or rectangle shapes for node shapes. The circle & rectangle shapes can be easily managed by the render engine, both for geometric or texture design. We also know that supporting different shapes within the same graph is possible, but more performance tests for that specific point must be done. Given that, and the idea that node shape settings must be independent from the render engine (and thus defined from the file format), we need to specify the general support of node shapes and complete the existing render support within the 3d engine.

General support
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- Store the shape settings in NodeData. Detail the way internal shape settings can be set and extended.
- Adapt Import modules to support this feature
- Adapt Export module to support this feature
- Propose the necessary options to let the user modify settings
- Adapt Project module to support saving/loading this feature

Visual support
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- Finish generic shape support in Visualization module and propose an API to extend shapes
- Develop spheres, rectangles and images and their internal parameters in the OpenGL engine