[MU3] Fix #322347 - ensure hidden rests in V2-4 don't move back to default position

[wizofaus]

Resolves: https://musescore.org/en/node/322347

Hiding V2-4 rests currently causes them to move back to the default/vertical center position on a stave, causing them to interfere with visible V1 elements. In general hiding items shouldn't cause the hidden item to move.

NB this is only a partial fix - the other improvement would be to ensure that hidden elements are drawn first (in a layer underneath everything else) so that visible ones can't be obscured by them.

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[wizofaus]

Fixing the z-order so that invisible elements are painted first, under visible elements, is easy with a simple change to Element::z() to always return 0 for invisible elements, or a relatively small change to paintElements( ) so that !visible() overrides z() when sorting.
Not sure which is best, though if we want invisible notes/rests drawn over staff lines, then it's probably better just to return a z() position of SYSTEM_DIVIDER * 100 (1400):

        if (!visible()) {
            return std::min(int(ElementType::SYSTEM_DIVIDER) * 100, int(type()) * 100);
        } else {
            return int(type()) * 100;
        }

(this ensures invisible rests are still drawn over invisible staff lines).