Links dropped on IO nodes disconnect

Links dropped on IO node "background", i.e. not precisely on an output slot, but still within the IO node hitbox for all other click events, are now behaving as if the link was dropped on the canvas.

- Cause: #4931

The PR does remove the exception (was unimplemented), but it shows a search menu to add a new node instead of connecting it to the virtual "add new slot" slot.

It is slightly confusing because the IO node backgrounds are transparent. This could be resolved by adding a hover border be shown when dragging any link over an IO node, and snapping to the "empty" slot. Then it would truly behave like a normal node.

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-4934-Links-dropped-on-IO-nodes-disconnect-24d6d73d36508113907fdef08e7383b0) by [Unito](https://www.unito.io)

Start by tracing the IO-node link-drop handling changed by #4931 and compare it with normal node hitbox behavior. The fix is done when dropping a link on an IO node background highlights and connects to the virtual empty/add-new-slot instead of opening the add-node menu or disconnecting.

Written by the indexing model from the issue text.