Using simple copies as layout placeholders

Right now, a full copy of a morph and all its submorphs is used as placeholder when hovering over a morph with a layouter. This causes unnecessary copying of attributes that are not relevant for the placeholder. Additionally, ticking scripts and connections will also get copied, causing unexpected behavior just be hovering over morphs.

This patch re-uses the grab shadow implementation which copies the shape of the morph and all its submorphs but does not consider connections, scripts, etc.