Confusing behaviour when attempting to delete output nodes or input nodes #14694
Description
Currently, input and output nodes cannot be deleted. There is an issue for adding support: #14450. However it is assigned to Future Release; in the interim I think our current behaviour is confusing. When the user attempts to delete nodes (e.g. with the Delete key), we proceed to delete only the deletable nodes, filtering out input and output nodes. This has the effect that, when selecting only an output node and pressing Delete, nothing happens. I think we should display a toast like "No deletable components selected" in this case--every handled key should have some visible effect, even if is a message indicating that the action isn't currently possible. I am not sure whether, in the case multiple nodes are selected but only some of them are (currently) deletable, we should display a warning, or just ignore the undeletable nodes as we do now.