move scene graph related code into a library

At the moment, inkscape source contains a fair amount of code related to the svg scene graph (roughly, all classes starting with SP (for SodiPodi)). This blueprint proposes to turn this svg graph related code into a library that we could call inkscene, so that this code could be re-used in other applications (not necessarily using a graphical user interface). This code being more widely used would mean more improvements and bug fixes.

The role of this library would be to provide an API to manipulate a 2d scene graph, and provide import from svg and export to svg file. What would remain in inkscape's code would be the part that handles the rendering of the scene graph. The challenge would be to manage to decouple inkscape's scene graph code from rendering code and from inkscape's interactive editor's specificities.