Updated library code to be able to store a group of nodes.
Added keyboard shortcuts:
- Del to delete selected nodes
- C to center the view
Modified paste action to paste at the center of the view.
New example (biohazard from webbzeug)
* Added a "constant wave" in the pattern node
* Updated graphEdit to detect and forbid loops
* Modified code that renders to texture to update a texture instead of returning one (so we avoid updating everything and rely on everything being updated automatically wrt textures)
* base library is loaded from filesystem (instead of package) if available
