Fix menu separator height

It's unclear why this regressed recently, given that the code has not been touched in a long time. But it's equally unclear how it worked in the first place, since the Kirigami.Separator item would have gotten automatically resized to the parent's height by being the background item. We need to hardcode the size to work around that, or else we'd need to stop overriding the background property and redo how this control works.

I am not 100% this is the best or most correct fix (though it does fix the problem for me) so I would appreciate reviews from QML experts like @mart or @ndavis.