Edit handles don't scale well when zooming

Reported in https://musescore.org/en/node/298940

1) add two beamed eighth notes to a score
2) click (3.4) or double-click (3.3.4) the beam to display handles
3) zoom in (eg, with Ctrl + mouse wheel)

Result: handles get "swallowed" by the beam. It seems they actually get smaller as the score gets bigger, although that could be an illusion. I suspect maybe at one time they got bigger in step with the score, and smaller when zooming out, and this was perceived as a bug that got fixed, but somehow, the fix seems not good either. We should make sure the handles don't get smaller, but also try to position them better.