make visu_osg easier to extend

[sbarthelemy]

Currently the visu is hard to extend because :

1. the WorldDrawer is quite complicated, and there is no well defined way to add custom data to it
   • we could dispatch its data in the osg graph (in UserData fields) instead
2. there is no way to add custom code (like: draw and update a force)
   • we could add arboris-defined callbacks
   • we could use osg-defined callback (this would make our code more similar to (and compatible with) the osgAnimate one)
3. the Viewer (especially the KeyboardHandler) is hard to extend
   • maybe we could support several KeyboardHandlers (simultaneously)
4. There is no well defined way for the viewer to modify the simulation
   • add support for kinematic (0-order) joints ?
   • allow to move subframes whose parent is ground ?

[sbarthelemy]

The visualization part of arboris will removed as soon as the C++ viewer is here.