Fix argument hint placement
- Lines were not counted properly for height, resulting in one line too many.
- It needs to be placed one line further up to be above the editing line. (Arguably, there could be special casing for when the completion widget has been moved to above the editing line, but arguably, that is overkill.)
I would have loved to replace some of the hard-coded pixel sizes, while at it, but did not quite figure them out.