The same wxRibbonBar can use multiple button bars with different icon
sizes, so 2 image lists are not enough. But OTOH there is no need to
distinguish between small and large images neither, so 2 may be also 1
too many.
Instead, use however many image lists we need, depending on the size.
For now, just store them in a vector and use linear search in it,
instead of using a map or, maybe, sorted vector, as we suppose there are
never going to be more than a couple of elements in this vector anyhow.
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