Fix #20547: Staff creation when instrument is selected

[SalvadorCorreia]

Previously, when adding a new staff under a selected instrument, it was created as non-selected. This caused confusion, particularly when hiding staffs, as selected staffs affected the newly created one, but not vice versa. To resolve this issue, I have updated the behavior so that a newly created staff is automatically selected if the instrument it is created for is selected. This ensures consistency and clarity in managing staffs, addressing the confusion experienced by users.

Resolves: #20547

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[cbjeukendrup]

Judging from AbstractInstrumentsPanelTreeItem::setIsSelected, we should perhaps go for a more general solution. What if you move this new code to AbstractInstrumentsPanelTreeItem::appendChild and AbstractInstrumentsPanelTreeItem::insertChild?

Of course, you can't literally move the code, because then it does the wrong thing... but you get the idea.

[Eism]

Yes, I also think we should take a more general solution. This should probably be the case, if the user adds a new staff or creates linked staff, then we should select the new item and deselect all other items.

[SalvadorCorreia]

I have currently followed @cbjeukendrup's instructions and believe to have made my solution more general.

[zacjansheski]

Tested on MacOS 14, Windows 11, Ubuntu 22.04.3. Approved https://github.com/musescore/MuseScore/issues/20547 FIXED