Currently, we just skipped the notification comparison, if the
notification had a raw icon attached. This is a bit counterintuitive.
Calculating a checksum of the raw icon's data is the solution.
For that we cache the pixel buffer and introduce a field, which saves
the current icon's id. The icon_id may be a path or a hash.
So you can compare two notifications by their icon_id field regardless
of their icon type by their icon_id field.
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