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Before this implementation, specific status via former mergeOverlayIcon() was greyed out
with the visibility.
With this commit, the developer has two options:
a) override mergeGreyableOverlayIcons, in which case the overlay will
be greyed out when the item is greyed out as per former mergeOverlayIcon()
b) override mergeColorfulOverlayIcons, in which case the overlay will
be superimposed after the icon is greyed out as it is the case for stardard
error and recompute flags.
