Previously there was already "draw_shape" property, but it was doing nothing. This commit renames the property to "display_shape". Furthermore, different shapes like SQUARE and DIAMOND are supported now. Currently, the shapes are drawn using the shader that also draws keyframes. In the future we might want to separate this. The new shapes are not used anywhere yet, but they can be used by addon developers and will probably be useful when we want to support different kinds node systems later. For example, different shapes can be used to distinguish between data and control flow. Differential Revision: https://developer.blender.org/D2829