Allow for custom invisible color

[XiaoMigros]

Resolves: #24054

Note this PR doesn't allow for a custom color of selected invisible items, those are currently the selection color at 40% opacity

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[wizofaus]

Tested this out, seems fine - I did originally think it'd be nice to have control over the degree of opacity for selected invisible notes but at least for me on my monitor I didn't feel any need to adjust it even after setting invisibleColor to #d0d0d0 (which is quite pale, but what I want - just enough to see there's something there but not getting in the way of what will be printed).

[wizofaus]

LGTM

[XiaoMigros]

I did originally think it'd be nice to have control over the degree of opacity for selected invisible notes

I believe that to be a separate issue - see #13652
Thanks for taking a look!

[Jojo-Schmitz]

Any reason not to merge this?

[zacjansheski]

In this PR, invisible accidentals are not displayed

video1422447981.mp4

[XiaoMigros]

Rebased to include #24306, should be good now!

[avvvvve]

Looks good! "Invisible color" is kind of a funny title, but there's not really a better way to put it without making it longer.

[avvvvve]

@XiaoMigros Zac just noticed that the invisible color is incorrectly being applied to the speaker icon in the sound flag button. That icon should just be black (#000000).