implement actualtext, artifact from recent graphicx

[xworld21]

Fix #2679 more or less as we discussed, but with more care about how the keys interact with each other:

• if artifact is true, @description is set to empty unless actualtext, alt are present
• if actualtext and alt are both present, save both as respectively description and aria-description

The latter might be slightly controversial. I played quickly with VoiceOver and it reads both alt and aria-description in sequence, with a proper pause (feels like "actualtext: alt"), which seems reasonable.

We could add a further heuristic that if actualtext and alt are equal, then just @description is enough.

I have added a test with all 8 combinations to clarify the intended semantics.