Expose Score spannerMap to plugins

[simonchatts]

Currently plugins cannot access voltas, since these are in the Score's spannerMap. This PR exposes both the spannerMap and the Volta class to the API. Plugins can either iterate all Score-level spanners using curScore.spanners, or filter down using, eg:

for (const volta of curScore.spannersOfType(Element.VOLTA)) {
    volta.lineColor = "#ff0000";
}

I've tested plugins using this sort of thing and they seem to work fine.

I've tried to follow the existing conventions: just safely expose the DOM directly with a minimal API layer. In particular, iterating the spannerMap from a plugin is quadratic (spannerListCount() and spannerListAt() both are O(n)) but the alternative seems to be building some sort of cache and knowing when to invalidate it, which is opening a can of worms. The scores I've tested have only had a small number of voltas, ties etc, and so exhibited no performance issues whatsoever.

I've assumed this will be a 4.7 thing in the new \since declarations.

The compiler warning fix 9da9c63 becomes an error with the other changes, hence its inclusion here.

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[XiaoMigros]

I am opposed to exposing voltas as a separate element in the API. All the newly added properties are retrievable with the API already (and if they aren't, then that is due to more systemic issues that should be solved by other means).
Exposing the spanner map is a welcome change, as those are some of the last elements that plugins have (up to now) had no easy way of finding.

[simonchatts]

I am opposed to exposing voltas as a separate element in the API.

Makes sense. I'll drop the voltas, and the separate spannersOfType method, and force-push an update. Thanks for the speedy comments!

[XiaoMigros]

For the record, volta endings can be accessed by the volta_ending property, listed in elements.h. beginText as well as endHookType cover all the remaining properties.

[simonchatts]

Changes pushed - should resolve all review comments so far. I've also updated the title to be more accurate. The updated example to turn voltas red is:

for (const spanner of curScore.spanners) {
    if (spanner.type === Element.VOLTA)) {
        spanner.lineColor = "#ff0000";
    }
}

[XiaoMigros]

After testing locally, this is great! A very useful addition to the API.

[manolo]

This fixes: #31061