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 combine node that assembles channels into colors
- Added an emboss node
- Added an export node that generates additional file when exporting the material
- Refactored convolution related code
* updated transform node (scale split into scale_x and scale_y, and added a repeat parameter)
* added blend modes (burn, dodge, lighten and darken)
* added a (very bad) blur node, based on generic convolution code
* rewrote normal_map node using convolution code
