A complete rewrite of SPCanvasItems (now Inkscape::CanvasItems) along with all the code they touched.

Canvas items were a mixture of semi-related structures and half-classes following the GObject model. This work replaces them with C++ classes, all derived from CanvasItem. It also standardizes how things such as geometry and style are handled.

No user visible changes should be seen except for a few knot/handle/node/etc. in a few LPE's which may use a differ shape.