Show artboard names in viewport

[0HyperCube]

Currently it is possible to rename an artboard by double clicking in the layer tree. However this change is not reflected in the viewport, where it is still titled "Artboard".

See impl GraphicElementRendered for Artboard for the artboard rendering code. This change necessitates the passing of the name string into the graph execution context, whereas currently it is just visible as metadata on the document graph and so not sent to the actual node implementations at all. If the ConstructLayerNode got the layer name as input then it could apply the name to the passed in GraphicElement, which would involve storing the string in the Artboard struct and then retrieving that during rendering.