Fix #15038: Recompute default stem length when hook is added

[asattely]

Resolves: https://github.com/musescore/MuseScore/issues/15038

The stems were poorly sized because the computation for stem length came before any beams were removed / hooks were added. Now there is a recalculation of stem length.

[its-not-nice]

Looks good; it is also necessary to re-render when stem directions change. When the first note of a) is split from the group, for example, we get b). (The stem direction should flip, which does happen when the layout is recomputed.)

[RomanPudashkin]

@asattely please pay attention to the failed utests