Fix flying gradual tempo changes in Palettes / Master Palette

[cbjeukendrup]

On every redraw of the palette cell (which included a call to layout()) they would be moved upwards a little bit.

Resolves: #15297

@mike-spa Do you think that the positioning of gradual tempo changes needs to be re-tested thoroughly after this change?

[cbjeukendrup]

Hm, yeah, it's indeed problematic for the placement below the staff...

[mike-spa]

Yeah, this is gonna need a deeper look. I remember why I did that change, but now that I look at it I'm thinking I should have done it differently. I'll see if I can find a better solution

[MarcSabatella]

FWIW, in the past there were a number of layout() functions that had hacks like checking parent to see if this was being laid in a score or on a palette, so things could be special-cased as needed. Not pretty and I'm not sure if that's still something used elsewhere or not, but this is exactly the sort of case where that could make sense.

[cbjeukendrup]

I think I'll close this, as Michele is already looking for a better solution.