Highlight BlockCode nodes in the 2D or 3D canvas

[dylanmccall]

When the user is editing a BlockCode node, it can be unclear what object in the scene they are affecting: nothing in the canvas appears as selected, because BlockCode isn't a CanvasItem. Previously, the plugin has tried to help by ensuring a BlockCode node's parent node is selected along with the BlockCode node itself, but this was removed in #227.

Instead, let's have the editor plugin add another node just for the BlockCode node's visual representation in the editor.