Remove Curve from the top of the Shape Tree

[rvodden]

At the moment all shapes inherit from the Shape object. The vast majority of shapes inherit from Shape via Curve. This is a legacy of the rendering logic being mixed up with the modelling logic; many of the shapes use Curve to be drawn. There is nothing wrong with using Curve to render primitive objects, but that should be a decision for the render (at present the DrawingTool implementation.

It would be great to refactor the model logic so that Curve isn't in the object hierarchy for shapes such as Circle and Line. This will mean changing the interface between the model and drawing_tool so that it knows how to draw more primitive shapes (i.e. add methods such as draw_line and draw_circle). This will mean that where rendering tools understand how to draw primitive shapes (like the <circle> tag in SVG) then we can take advantage of that. For the scope of this issue we can continue to reuse the curve drawing logic, it just needs to move to drawing_tool.