Simplified Infrastructure for Rendering Custom SceneGraph Shaders

This introduces a new scene graph node type in the primitives module, ShaderNode, which greatly simplifies using custom shaders for rendering items. ShaderNode can be created in updatePaintNode(), pointed at a shader and given some uniform data, after which it should just render. As an example, ShadowedRectangle and related types have been ported, greatly reducing the amount of duplicate code for these types.

It also ports Icon to use the new shader system.