Fix #305265: Bracket added from palette only spans one staff regardless of selection

[IsaacWeiss]

Resolves: https://musescore.org/en/node/305265

Small usability/workflow improvement so that newly added bracket spans all selected staves. Previously, only the top staff of the selection would get the bracket.

Submitted against 3.x, but should also be cherry-picked to master.

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[Jojo-Schmitz]

needed in master too