Add "Recalculating Layout" dialogue box after reordering instruments

[zacjansheski]

Your idea

It would be helpful to add a dialogue box that says "Recalculating Layout" while MuseScore is working on such a task

Problem to be solved

After dragging and dropping an instrument to re-order on a large score, it can take a while for the score to update and the user doesn't have any message, this can lead them to think there is a freeze.

Prior art

Something similar to this:

Additional context

Discussed internally while implementing this: https://github.com/musescore/MuseScore/pull/19907

[avvvvve]

Same goes for hiding/unhiding instruments in a score with lots of instruments! Good catch.

We could go for a slightly less technical-sounding message:

• "Redrawing score"
• "Updating score layout"

Or—and adding this to my to-dos—we could have a more generic loading animation for actions that cause the "freeze". Perhaps a short accent-colored loading bar that spans the top of the canvas pane.

[avvvvve]

Note: We should update the Instruments UI immediately while the score is finishing processing